diff --git a/app/assets/stylesheets/shared/blocks/_piyo-companion.css b/app/assets/stylesheets/shared/blocks/_piyo-companion.css
index 026480c4d55..b29cde6c99c 100644
--- a/app/assets/stylesheets/shared/blocks/_piyo-companion.css
+++ b/app/assets/stylesheets/shared/blocks/_piyo-companion.css
@@ -11,68 +11,164 @@
   cursor: pointer;
   padding: 0;
   position: relative;
-  transition: transform 0.15s;
-}
-
-.piyo-companion__button:hover {
-  transform: scale(1.1);
 }
 
 .piyo-companion__image {
-  width: 56px;
-  height: 56px;
-  filter: drop-shadow(0 2px 4px rgba(0, 0, 0, 0.15));
+  width: 64px;
+  height: 64px;
 }
 
 .piyo-companion__bubble {
   position: absolute;
-  bottom: 64px;
+  bottom: 72px;
   right: 0;
-  background: var(--base, #fff);
-  border: 1px solid var(--border, #e0e0e0);
+  background: #fff;
+  border: 1px solid #e0e0e0;
   border-radius: 12px;
-  padding: 0.625rem 0.875rem;
-  max-width: 220px;
-  box-shadow: 0 2px 12px rgba(0, 0, 0, 0.1);
-  animation: piyo-bubble-in 0.2s ease-out;
+  padding: 0.75rem 1rem;
+  max-width: 240px;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);
 }
 
 .piyo-companion__bubble::after {
   content: '';
   position: absolute;
-  bottom: -7px;
-  right: 20px;
+  bottom: -8px;
+  right: 24px;
   width: 0;
   height: 0;
-  border-left: 7px solid transparent;
-  border-right: 7px solid transparent;
-  border-top: 7px solid var(--base, #fff);
+  border-left: 8px solid transparent;
+  border-right: 8px solid transparent;
+  border-top: 8px solid #fff;
 }
 
 .piyo-companion__message {
   margin: 0;
+  font-size: 0.875rem;
+  line-height: 1.4;
+}
+
+.piyo-chat-panel {
+  position: absolute;
+  bottom: 72px;
+  right: 0;
+  width: 320px;
+  max-height: 400px;
+  background: #fff;
+  border: 1px solid #e0e0e0;
+  border-radius: 12px;
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.12);
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+}
+
+.piyo-chat-panel__header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 0.75rem 1rem;
+  border-bottom: 1px solid #e0e0e0;
+  font-weight: bold;
+  font-size: 0.875rem;
+}
+
+.piyo-chat-panel__close {
+  background: none;
+  border: none;
+  cursor: pointer;
+  font-size: 1.25rem;
+  color: #666;
+  padding: 0;
+  line-height: 1;
+}
+
+.piyo-chat-panel__close:hover {
+  color: #333;
+}
+
+.piyo-chat-panel__messages {
+  flex: 1;
+  overflow-y: auto;
+  padding: 0.75rem;
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+  min-height: 200px;
+}
+
+.piyo-chat-panel__msg {
+  max-width: 85%;
+  padding: 0.5rem 0.75rem;
+  border-radius: 12px;
   font-size: 0.8125rem;
   line-height: 1.5;
+  word-break: break-word;
+}
+
+.piyo-chat-panel__msg--user {
+  align-self: flex-end;
+  background: #dbeafe;
+  color: #1e3a5f;
+}
+
+.piyo-chat-panel__msg--assistant {
+  align-self: flex-start;
+  background: #f3f4f6;
+  color: #333;
+}
+
+.piyo-chat-panel__input {
+  display: flex;
+  gap: 0.5rem;
+  padding: 0.75rem;
+  border-top: 1px solid #e0e0e0;
+}
+
+.piyo-chat-panel__textarea {
+  flex: 1;
+  border: 1px solid #d1d5db;
+  border-radius: 8px;
+  padding: 0.5rem;
+  font-size: 0.8125rem;
+  resize: none;
+  min-height: 2.5rem;
+  max-height: 5rem;
+  font-family: inherit;
+}
+
+.piyo-chat-panel__textarea:focus {
+  outline: none;
+  border-color: #60a5fa;
+}
+
+.piyo-chat-panel__send {
+  background: #3b82f6;
+  color: #fff;
+  border: none;
+  border-radius: 8px;
+  padding: 0.5rem 0.75rem;
+  font-size: 0.8125rem;
+  cursor: pointer;
+  white-space: nowrap;
+}
+
+.piyo-chat-panel__send:hover {
+  background: #2563eb;
+}
+
+.piyo-chat-panel__send:disabled {
+  background: #93c5fd;
+  cursor: not-allowed;
 }
 
 .piyo-companion__badge {
   position: absolute;
-  top: -3px;
-  right: -3px;
-  width: 10px;
-  height: 10px;
-  background: var(--danger, #ef4444);
+  top: -4px;
+  right: -4px;
+  width: 12px;
+  height: 12px;
+  background: #ef4444;
   border-radius: 50%;
-  border: 2px solid var(--base, #fff);
-}
-
-@keyframes piyo-bubble-in {
-  from {
-    opacity: 0;
-    transform: translateY(4px);
-  }
-  to {
-    opacity: 1;
-    transform: translateY(0);
-  }
+  border: 2px solid #fff;
 }
diff --git a/app/controllers/api/textbooks/piyo_chat_messages_controller.rb b/app/controllers/api/textbooks/piyo_chat_messages_controller.rb
new file mode 100644
index 00000000000..a4818411f39
--- /dev/null
+++ b/app/controllers/api/textbooks/piyo_chat_messages_controller.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+class API::Textbooks::PiyoChatMessagesController < API::BaseController
+  include TextbookFeatureGuard
+  before_action :require_textbook_enabled
+
+  def index
+    messages = current_user.piyo_chat_messages
+                           .where(textbook_section_id: params[:section_id])
+                           .order(created_at: :asc)
+
+    render json: messages.map { |m| message_json(m) }
+  end
+
+  def create
+    section = Textbook::Section.find(params[:section_id])
+
+    user_message = current_user.piyo_chat_messages.create!(
+      textbook_section_id: section.id,
+      role: 'user',
+      content: params[:content]
+    )
+
+    response_text = PiyoChatService.respond(
+      user: current_user,
+      section: section,
+      message: params[:content]
+    )
+
+    assistant_message = current_user.piyo_chat_messages.create!(
+      textbook_section_id: section.id,
+      role: 'assistant',
+      content: response_text
+    )
+
+    render json: {
+      user_message: message_json(user_message),
+      assistant_message: message_json(assistant_message)
+    }, status: :created
+  end
+
+  private
+
+  def message_json(message)
+    {
+      id: message.id,
+      role: message.role,
+      content: message.content,
+      created_at: message.created_at
+    }
+  end
+end
diff --git a/app/javascript/controllers/piyo_chat_controller.js b/app/javascript/controllers/piyo_chat_controller.js
new file mode 100644
index 00000000000..9a1eaa4dc9a
--- /dev/null
+++ b/app/javascript/controllers/piyo_chat_controller.js
@@ -0,0 +1,129 @@
+import { Controller } from 'stimulus'
+
+export default class extends Controller {
+  static targets = ['panel', 'messages', 'input', 'sendButton', 'badge']
+  static values = { sectionId: Number, open: { type: Boolean, default: false } }
+
+  connect() {
+    this.loadHistory()
+  }
+
+  toggle() {
+    this.openValue = !this.openValue
+    this.panelTarget.classList.toggle('is-hidden', !this.openValue)
+
+    if (this.openValue) {
+      this.hideBadge()
+      this.scrollToBottom()
+      this.inputTarget.focus()
+    }
+  }
+
+  close() {
+    this.openValue = false
+    this.panelTarget.classList.add('is-hidden')
+  }
+
+  async send() {
+    const content = this.inputTarget.value.trim()
+    if (!content) return
+
+    this.inputTarget.value = ''
+    this.appendMessage('user', content)
+    this.setLoading(true)
+
+    try {
+      const response = await fetch('/api/textbooks/piyo_chat_messages', {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json',
+          'X-CSRF-Token': this.csrfToken
+        },
+        body: JSON.stringify({
+          section_id: this.sectionIdValue,
+          content: content
+        })
+      })
+
+      if (!response.ok) throw new Error('送信に失敗しました')
+
+      const data = await response.json()
+      this.appendMessage('assistant', data.assistant_message.content)
+    } catch (error) {
+      this.appendMessage('assistant', 'ごめんなさい、エラーが発生しました。もう一度試してみてください。')
+    } finally {
+      this.setLoading(false)
+    }
+  }
+
+  onKeydown(event) {
+    if (event.key === 'Enter' && !event.shiftKey) {
+      event.preventDefault()
+      this.send()
+    }
+  }
+
+  async loadHistory() {
+    try {
+      const response = await fetch(
+        `/api/textbooks/piyo_chat_messages?section_id=${this.sectionIdValue}`
+      )
+      if (!response.ok) return
+
+      const messages = await response.json()
+      messages.forEach((msg) => this.appendMessage(msg.role, msg.content))
+    } catch {
+      // 履歴読み込み失敗は無視
+    }
+  }
+
+  appendMessage(role, content) {
+    const el = document.createElement('div')
+    el.className = `piyo-chat-message piyo-chat-message--${role}`
+    el.textContent = content
+    this.messagesTarget.appendChild(el)
+    this.scrollToBottom()
+
+    if (!this.openValue && role === 'assistant') {
+      this.showBadge()
+    }
+  }
+
+  setLoading(loading) {
+    this.sendButtonTarget.disabled = loading
+
+    if (loading) {
+      this.loadingEl = document.createElement('div')
+      this.loadingEl.className = 'piyo-chat-message piyo-chat-message--assistant piyo-chat-message--loading'
+      this.loadingEl.textContent = '...'
+      this.messagesTarget.appendChild(this.loadingEl)
+      this.scrollToBottom()
+    } else if (this.loadingEl) {
+      this.loadingEl.remove()
+      this.loadingEl = null
+    }
+  }
+
+  scrollToBottom() {
+    requestAnimationFrame(() => {
+      this.messagesTarget.scrollTop = this.messagesTarget.scrollHeight
+    })
+  }
+
+  showBadge() {
+    if (this.hasBadgeTarget) {
+      this.badgeTarget.classList.remove('is-hidden')
+    }
+  }
+
+  hideBadge() {
+    if (this.hasBadgeTarget) {
+      this.badgeTarget.classList.add('is-hidden')
+    }
+  }
+
+  get csrfToken() {
+    const meta = document.querySelector('meta[name="csrf-token"]')
+    return meta ? meta.content : ''
+  }
+}
diff --git a/app/models/piyo_chat_message.rb b/app/models/piyo_chat_message.rb
new file mode 100644
index 00000000000..4269f5bafc2
--- /dev/null
+++ b/app/models/piyo_chat_message.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class PiyoChatMessage < ApplicationRecord
+  belongs_to :user
+  belongs_to :section, class_name: 'Textbook::Section', foreign_key: 'textbook_section_id', inverse_of: :piyo_chat_messages
+
+  validates :role, presence: true, inclusion: { in: %w[user assistant] }
+  validates :content, presence: true
+
+  scope :recent, -> { order(created_at: :asc) }
+end
diff --git a/app/models/textbook/section.rb b/app/models/textbook/section.rb
index d1dddd6110c..9932a14f729 100644
--- a/app/models/textbook/section.rb
+++ b/app/models/textbook/section.rb
@@ -6,6 +6,7 @@ class Textbook::Section < ApplicationRecord
   belongs_to :chapter, class_name: 'Textbook::Chapter', foreign_key: 'textbook_chapter_id', inverse_of: :sections
   has_many :reading_progresses, foreign_key: 'textbook_section_id', dependent: :destroy, inverse_of: :section
   has_many :term_explanations, foreign_key: 'textbook_section_id', dependent: :destroy, inverse_of: :section
+  has_many :piyo_chat_messages, foreign_key: 'textbook_section_id', dependent: :destroy, inverse_of: :section
 
   validates :title, presence: true
   validates :body, presence: true
diff --git a/app/models/user.rb b/app/models/user.rb
index eafe9657444..5dc800ae9a0 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -93,6 +93,8 @@ class User < ApplicationRecord # rubocop:todo Metrics/ClassLength
   belongs_to :course
   has_many :learnings, dependent: :destroy
   has_many :reading_progresses, dependent: :destroy
+  has_many :piyo_chat_messages, dependent: :destroy
+  has_many :piyo_chat_messages, dependent: :destroy
   has_many :pages, dependent: :destroy
   has_many :comments, dependent: :destroy
   has_many :reports, dependent: :destroy
diff --git a/app/services/piyo_chat_service.rb b/app/services/piyo_chat_service.rb
new file mode 100644
index 00000000000..79da8626c5d
--- /dev/null
+++ b/app/services/piyo_chat_service.rb
@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+class PiyoChatService
+  SECTION_PROMPT = <<~PROMPT
+    ## 現在のコンテキスト
+    ユーザーは教科書のセクションを読んでいます。
+    このセクションの内容に基づいて回答してください。
+    回答は簡潔にわかりやすく、セクションの範囲内で説明してください。
+    脱線せず、セクションの学習目標に沿った回答を心がけてください。
+  PROMPT
+
+  class << self
+    def respond(user:, section:, message:)
+      chat = RubyLLM.chat(model: model_name)
+      chat.with_instructions(build_prompt(section))
+
+      history = recent_history(user, section)
+      history.each do |msg|
+        chat.ask(msg.content) if msg.role == 'user'
+      end
+
+      result = chat.ask(message)
+      result.content.presence || 'ごめんなさい、うまく回答できませんでした。メンターに聞いてみてください。'
+    rescue StandardError => e
+      Rails.logger.error("PiyoChatService error: #{e.message}")
+      'ごめんなさい、エラーが発生しました。もう一度試してみてください。'
+    end
+
+    private
+
+    def model_name
+      ENV.fetch('PJORD_LLM_MODEL', 'gpt-5-mini')
+    end
+
+    def build_prompt(section)
+      parts = [Pjord::SYSTEM_PROMPT, SECTION_PROMPT]
+      parts << textbook_info(section)
+      parts << goals_section(section) if section.goals.present? && section.goals.any?(&:present?)
+      parts << terms_section(section) if section.key_terms.present? && section.key_terms.any?(&:present?)
+      parts.join("\n\n")
+    end
+
+    def textbook_info(section)
+      "## 教科書情報\n- 教科書: #{section.chapter.textbook.title}\n- 章: #{section.chapter.title}\n- セクション: #{section.title}"
+    end
+
+    def goals_section(section)
+      "## 学習目標\n#{section.goals.select(&:present?).map { |g| "- #{g}" }.join("\n")}"
+    end
+
+    def terms_section(section)
+      "## 重要用語\n#{section.key_terms.select(&:present?).join(', ')}"
+    end
+
+    def recent_history(user, section)
+      PiyoChatMessage.where(user: user, textbook_section_id: section.id)
+                     .order(created_at: :asc)
+                     .last(10)
+    end
+  end
+end
diff --git a/app/views/shared/_piyo_companion.html.slim b/app/views/shared/_piyo_companion.html.slim
index 235e0d4c728..a210704c928 100644
--- a/app/views/shared/_piyo_companion.html.slim
+++ b/app/views/shared/_piyo_companion.html.slim
@@ -1,10 +1,141 @@
 - section_id = local_assigns[:section_id]
 
-.piyo-companion data-controller='piyo-companion'
+.piyo-companion data-controller="piyo-companion #{section_id ? 'piyo-chat' : ''}" data-piyo-chat-section-id-value=section_id
+  /! チャットパネル（セクション閲覧時のみ）
+  - if section_id
+    .piyo-chat-panel.is-hidden data-piyo-chat-target='panel'
+      .piyo-chat-panel__header
+        span.piyo-chat-panel__title 🐥 ピヨルドに質問
+        button.piyo-chat-panel__close type='button' data-action='click->piyo-chat#close'
+          i.fa-solid.fa-xmark
+      .piyo-chat-panel__messages data-piyo-chat-target='messages'
+      .piyo-chat-panel__input
+        textarea.piyo-chat-panel__textarea data-piyo-chat-target='input' data-action='keydown->piyo-chat#onKeydown' placeholder='質問を入力...' rows='2'
+        button.piyo-chat-panel__send data-piyo-chat-target='sendButton' data-action='click->piyo-chat#send' type='button'
+          i.fa-solid.fa-paper-plane
+
   /! 吹き出し
   .piyo-companion__bubble.is-hidden data-piyo-companion-target='bubble'
     p.piyo-companion__message data-piyo-companion-target='message'
 
   /! ピヨルドボタン
-  button.piyo-companion__button type='button' data-action='click->piyo-companion#dismiss'
-    = image_tag 'shared/piyo.svg', alt: 'ピヨルド', class: 'piyo-companion__image'
+  - if section_id
+    button.piyo-companion__button type='button' data-action='click->piyo-chat#toggle'
+      .piyo-companion__badge.is-hidden data-piyo-chat-target='badge'
+      = image_tag 'shared/piyo.svg', alt: 'ピヨルド', class: 'piyo-companion__image'
+  - else
+    button.piyo-companion__button type='button' data-action='click->piyo-companion#dismiss'
+      = image_tag 'shared/piyo.svg', alt: 'ピヨルド', class: 'piyo-companion__image'
+
+css:
+  .piyo-chat-panel {
+    position: fixed;
+    bottom: 80px;
+    right: 16px;
+    width: 340px;
+    max-height: 480px;
+    background: var(--color-white, #fff);
+    border: 1px solid var(--color-border, #ddd);
+    border-radius: 12px;
+    box-shadow: 0 4px 24px rgba(0, 0, 0, 0.12);
+    display: flex;
+    flex-direction: column;
+    z-index: 1001;
+    overflow: hidden;
+  }
+  .piyo-chat-panel.is-hidden { display: none; }
+
+  .piyo-chat-panel__header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: 12px 16px;
+    border-bottom: 1px solid var(--color-border, #ddd);
+    background: var(--color-bg-light, #fafafa);
+  }
+  .piyo-chat-panel__title { font-weight: 600; font-size: 14px; }
+  .piyo-chat-panel__close {
+    background: none;
+    border: none;
+    cursor: pointer;
+    color: var(--color-muted, #999);
+    font-size: 16px;
+  }
+
+  .piyo-chat-panel__messages {
+    flex: 1;
+    overflow-y: auto;
+    padding: 12px;
+    min-height: 200px;
+    max-height: 320px;
+  }
+
+  .piyo-chat-message {
+    margin-bottom: 8px;
+    padding: 8px 12px;
+    border-radius: 12px;
+    font-size: 14px;
+    line-height: 1.5;
+    max-width: 85%;
+    word-wrap: break-word;
+    white-space: pre-wrap;
+  }
+  .piyo-chat-message--user {
+    background: var(--color-primary, #4a90d9);
+    color: #fff;
+    margin-left: auto;
+    border-bottom-right-radius: 4px;
+  }
+  .piyo-chat-message--assistant {
+    background: var(--color-bg-light, #f0f0f0);
+    color: var(--color-text, #333);
+    margin-right: auto;
+    border-bottom-left-radius: 4px;
+  }
+  .piyo-chat-message--loading { opacity: 0.6; }
+
+  .piyo-chat-panel__input {
+    display: flex;
+    align-items: flex-end;
+    gap: 8px;
+    padding: 8px 12px;
+    border-top: 1px solid var(--color-border, #ddd);
+  }
+  .piyo-chat-panel__textarea {
+    flex: 1;
+    border: 1px solid var(--color-border, #ddd);
+    border-radius: 8px;
+    padding: 8px;
+    font-size: 14px;
+    resize: none;
+    font-family: inherit;
+  }
+  .piyo-chat-panel__textarea:focus {
+    outline: none;
+    border-color: var(--color-primary, #4a90d9);
+  }
+  .piyo-chat-panel__send {
+    background: var(--color-primary, #4a90d9);
+    color: #fff;
+    border: none;
+    border-radius: 8px;
+    width: 36px;
+    height: 36px;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+  }
+  .piyo-chat-panel__send:disabled { opacity: 0.5; cursor: not-allowed; }
+
+  .piyo-companion__badge {
+    position: absolute;
+    top: -2px;
+    right: -2px;
+    width: 12px;
+    height: 12px;
+    background: #e74c3c;
+    border-radius: 50%;
+    border: 2px solid #fff;
+  }
+  .piyo-companion__badge.is-hidden { display: none; }
diff --git a/app/views/textbooks/sections/show.html.slim b/app/views/textbooks/sections/show.html.slim
index 735e94a9f39..b15772fe4a0 100644
--- a/app/views/textbooks/sections/show.html.slim
+++ b/app/views/textbooks/sections/show.html.slim
@@ -1,14 +1,5 @@
 - title "#{@textbook.title} - #{@section.title}"
 
-ruby:
-  sections = @chapter.sections.to_a
-  current_index = sections.index(@section)
-  progress_percent = ((current_index + 1).to_f / sections.size * 100).round
-  all_sections = @textbook.chapters.flat_map(&:sections)
-  global_index = all_sections.index(@section)
-  prev_section = global_index.positive? ? all_sections[global_index - 1] : nil
-  next_section = global_index < all_sections.size - 1 ? all_sections[global_index + 1] : nil
-
 main.page-main
   header.page-main-header
     .container
@@ -24,65 +15,59 @@ main.page-main
                   | 目次に戻る
   hr.a-border
   .page-body
-    .page-body__inner.has-side-nav
-      .container.is-md
-        /! パンくず
-        nav.textbook-breadcrumb
-          = link_to @textbook.title, textbook_path(@textbook)
-          |  ›
-          = " 第#{@chapter.position}章 #{@chapter.title}"
-          |  ›
-          span = " #{current_index + 1}/#{sections.size}"
-
-        /! 学習目標
-        - if @section.goals.present? && @section.goals.any?(&:present?)
-          .a-card.is-sm
-            header.card-header.is-sm
-              h2.card-header__title
-                i.fa-solid.fa-bullseye
-                |  このセクションの目標
-            .card-body
-              ul.card-body__items
-                - @section.goals.each do |goal|
-                  - next if goal.blank?
-                  li.card-body__item
-                    = goal
+    .container.is-md
+      /! Progress bar
+      ruby:
+        sections = @chapter.sections.to_a
+        current_index = sections.index(@section)
+        progress_percent = ((current_index + 1).to_f / sections.size * 100).round
+      .mb-2
+        .flex.items-center.gap-2
+          span.text-muted
+            = "#{@chapter.title} (#{current_index + 1}/#{sections.size})"
+        .progress-bar.mt-1
+          .progress-bar__fill style="width: #{progress_percent}%"
 
-        /! 本文
-        .a-card data-controller='reading-progress' data-reading-progress-section-id-value="#{@section.id}" data-reading-progress-total-blocks-value='0'
+      /! Section goals
+      - if @section.goals.present? && @section.goals.any?(&:present?)
+        .a-card.mb-2
+          header.card-header
+            h2.card-header__title
+              i.fa-solid.fa-bullseye.mr-1
+              | このセクションの目標
+          hr.a-border-tint
           .card-body
-            .card-body__description
-              .a-long-text.is-md.js-markdown-view
-                = textbook_section_body(@section)
+            ul.card-body__items
+              - @section.goals.each do |goal|
+                - next if goal.blank?
+                li.card-body__item
+                  = goal
+
+      /! Main content
+      .a-card data-controller='reading-progress' data-reading-progress-section-id-value="#{@section.id}" data-reading-progress-total-blocks-value='0'
+        .card-body
+          .card-body__description
+            .a-long-text.is-md.js-markdown-view
+              = textbook_section_body(@section)
 
-        /! 前後ナビ
-        .page-content-prev-next
-          .page-content-prev-next__item
-            - if prev_section
-              = link_to textbook_chapter_section_path(@textbook, prev_section.chapter, prev_section), class: 'page-content-prev-next__item-link is-prev' do
-                i.fa-solid.fa-angle-left
-                |  前へ
-          .page-content-prev-next__item
-            - if next_section
-              = link_to textbook_chapter_section_path(@textbook, next_section.chapter, next_section), class: 'page-content-prev-next__item-link is-next' do
-                | 次へ
-                i.fa-solid.fa-angle-right
+      /! Previous/Next navigation
+      ruby:
+        all_sections = @textbook.chapters.flat_map(&:sections)
+        global_index = all_sections.index(@section)
+        prev_section = global_index.positive? ? all_sections[global_index - 1] : nil
+        next_section = global_index < all_sections.size - 1 ? all_sections[global_index + 1] : nil
 
-      /! サイドナビ（章のセクション一覧）
-      nav.a-side-nav
-        .a-side-nav__inner
-          header.a-side-nav__header
-            h2.a-side-nav__title
-              = link_to textbook_path(@textbook), class: 'a-side-nav__title-link' do
-                = "第#{@chapter.position}章 #{@chapter.title}"
-          hr.a-border
-          .a-side-nav__body
-            ul.a-side-nav__items
-              - sections.each do |section|
-                li.a-side-nav__item class=("#{'is-current' if section == @section}")
-                  = link_to textbook_chapter_section_path(@textbook, @chapter, section), class: 'a-side-nav__item-link' do
-                    span.a-side-nav__item-link-inner
-                      = section.title
+      .page-content-prev-next.mt-2
+        .page-content-prev-next__item
+          - if prev_section
+            = link_to textbook_chapter_section_path(@textbook, prev_section.chapter, prev_section), class: 'page-content-prev-next__item-link is-prev' do
+              i.fa-solid.fa-angle-left.mr-1
+              = prev_section.title
+        .page-content-prev-next__item
+          - if next_section
+            = link_to textbook_chapter_section_path(@textbook, next_section.chapter, next_section), class: 'page-content-prev-next__item-link is-next' do
+              = next_section.title
+              i.fa-solid.fa-angle-right.ml-1
 
-  /! ピヨルド
+  /! Piyo companion
   = render 'shared/piyo_companion', section_id: @section.id
diff --git a/config/routes/textbook.rb b/config/routes/textbook.rb
index 78753561764..f7269a953cd 100644
--- a/config/routes/textbook.rb
+++ b/config/routes/textbook.rb
@@ -11,6 +11,8 @@
     namespace :textbooks do
       resources :reading_progresses, only: %i[create update]
       resources :term_explanations, only: %i[show]
+      resources :piyo_chat_messages, only: %i[index create]
+      resources :piyo_chat_messages, only: %i[index create]
     end
   end
 end
diff --git a/db/migrate/20260329000000_create_piyo_chat_messages.rb b/db/migrate/20260329000000_create_piyo_chat_messages.rb
new file mode 100644
index 00000000000..9e2ea3fe482
--- /dev/null
+++ b/db/migrate/20260329000000_create_piyo_chat_messages.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class CreatePiyoChatMessages < ActiveRecord::Migration[8.0]
+  def change
+    create_table :piyo_chat_messages do |t|
+      t.references :user, null: false, foreign_key: true
+      t.references :textbook_section, null: false, foreign_key: { to_table: :textbook_sections }
+      t.string :role, null: false
+      t.text :content, null: false
+
+      t.timestamps
+    end
+
+    add_index :piyo_chat_messages, %i[user_id textbook_section_id created_at],
+              name: 'index_piyo_chat_messages_on_user_section_created'
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index fc5bfd87a48..0936cde80df 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -593,6 +593,18 @@
     t.index ["user_id"], name: "index_participations_on_user_id"
   end
 
+  create_table "piyo_chat_messages", force: :cascade do |t|
+    t.text "content", null: false
+    t.datetime "created_at", null: false
+    t.string "role", null: false
+    t.bigint "textbook_section_id", null: false
+    t.datetime "updated_at", null: false
+    t.bigint "user_id", null: false
+    t.index ["textbook_section_id"], name: "index_piyo_chat_messages_on_textbook_section_id"
+    t.index ["user_id", "textbook_section_id", "created_at"], name: "index_piyo_chat_messages_on_user_section_created"
+    t.index ["user_id"], name: "index_piyo_chat_messages_on_user_id"
+  end
+
   create_table "practices", id: :serial, force: :cascade do |t|
     t.integer "category_id"
     t.datetime "created_at", precision: nil
@@ -1234,6 +1246,8 @@
   add_foreign_key "pair_works", "users", column: "buddy_id"
   add_foreign_key "participations", "events"
   add_foreign_key "participations", "users"
+  add_foreign_key "piyo_chat_messages", "textbook_sections"
+  add_foreign_key "piyo_chat_messages", "users"
   add_foreign_key "practices", "practices", column: "source_id"
   add_foreign_key "practices_books", "books"
   add_foreign_key "practices_books", "practices"
diff --git a/test/fixtures/piyo_chat_messages.yml b/test/fixtures/piyo_chat_messages.yml
new file mode 100644
index 00000000000..c331bce68c4
--- /dev/null
+++ b/test/fixtures/piyo_chat_messages.yml
@@ -0,0 +1,11 @@
+user_question:
+  user: hatsuno
+  textbook_section_id: <%= ActiveRecord::FixtureSet.identify(:section_variables) %>
+  role: user
+  content: Linuxって何ですか？
+
+assistant_answer:
+  user: hatsuno
+  textbook_section_id: <%= ActiveRecord::FixtureSet.identify(:section_variables) %>
+  role: assistant
+  content: Linuxはオープンソースのオペレーティングシステムだよ！
diff --git a/test/models/piyo_chat_message_test.rb b/test/models/piyo_chat_message_test.rb
new file mode 100644
index 00000000000..2cabaa11f19
--- /dev/null
+++ b/test/models/piyo_chat_message_test.rb
@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class PiyoChatMessageTest < ActiveSupport::TestCase
+  test 'valid with user, section, role, and content' do
+    message = piyo_chat_messages(:user_question)
+    assert message.valid?
+  end
+
+  test 'role must be user or assistant' do
+    message = piyo_chat_messages(:user_question)
+    message.role = 'system'
+    assert_not message.valid?
+  end
+
+  test 'content is required' do
+    message = piyo_chat_messages(:user_question)
+    message.content = ''
+    assert_not message.valid?
+  end
+
+  test 'belongs to user' do
+    message = piyo_chat_messages(:user_question)
+    assert_equal users(:hatsuno), message.user
+  end
+
+  test 'belongs to section' do
+    message = piyo_chat_messages(:user_question)
+    assert_equal textbook_sections(:section_variables), message.section
+  end
+end
diff --git a/test/services/piyo_chat_service_test.rb b/test/services/piyo_chat_service_test.rb
new file mode 100644
index 00000000000..b1704c600fe
--- /dev/null
+++ b/test/services/piyo_chat_service_test.rb
@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class PiyoChatServiceTest < ActiveSupport::TestCase
+  test 'respond returns assistant text' do
+    section = textbook_sections(:section_variables)
+    user = users(:hatsuno)
+
+    mock_result = OpenStruct.new(content: 'テスト回答です')
+    mock_chat = Object.new
+    mock_chat.define_singleton_method(:with_instructions) { |_| self }
+    mock_chat.define_singleton_method(:ask) { |_| mock_result }
+
+    RubyLLM.stub(:chat, mock_chat) do
+      result = PiyoChatService.respond(user: user, section: section, message: 'テスト質問')
+      assert_equal 'テスト回答です', result
+    end
+  end
+
+  test 'respond returns fallback on error' do
+    section = textbook_sections(:section_variables)
+    user = users(:hatsuno)
+
+    RubyLLM.stub(:chat, ->(*) { raise StandardError, 'API error' }) do
+      result = PiyoChatService.respond(user: user, section: section, message: 'テスト')
+      assert_includes result, 'エラーが発生しました'
+    end
+  end
+end