diff --git a/frontend/src/pages/theatres/TheatreDetailPage.test.tsx b/frontend/src/pages/theatres/TheatreDetailPage.test.tsx
index 7adab24..7f7bf12 100644
--- a/frontend/src/pages/theatres/TheatreDetailPage.test.tsx
+++ b/frontend/src/pages/theatres/TheatreDetailPage.test.tsx
@@ -1,9 +1,10 @@
-import { act, fireEvent, screen, waitFor } from '@testing-library/react'
+import { act, fireEvent, screen, waitFor, within } from '@testing-library/react'
 import { Route, Routes } from 'react-router-dom'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import TheatreDetailPage from '@/pages/theatres/TheatreDetailPage'
 import { buildPost,
+	 buildTag,
 	 buildTheatre,
 	 buildTheatreComment,
 	 buildTheatreInfo,
@@ -96,6 +97,9 @@ const renderPage = (user = buildUser ({ id: 1, role: 'member' })) =>
       { route: '/theatres/7' },
   )
 
+const tagSection = (): HTMLElement =>
+  screen.getAllByRole ('heading', { name: 'タグ' })[0].closest ('section')!
+
 const mockDefaultApi = () => {
   api.apiGet.mockImplementation ((path: string) => {
     switch (path)
@@ -245,6 +249,54 @@ describe ('TheatreDetailPage', () => {
     expect (postEmbed.seek).not.toHaveBeenCalledWith (0)
   })
 
+  it ('shows child tags from the post tag tree in both vertical and horizontal layouts', async () => {
+    const childTag = buildTag ({ id: 12, name: '子タグ', category: 'general' })
+    const parentTag = buildTag ({
+      id:       11,
+      name:     '親タグ',
+      category: 'general',
+      children: [childTag],
+    })
+    const duplicateParentTag = buildTag ({
+      id:       13,
+      name:     '別親タグ',
+      category: 'general',
+      children: [childTag],
+    })
+
+    postsApi.fetchPost.mockResolvedValueOnce (buildPost ({
+      ...currentPost,
+      tags: [parentTag, duplicateParentTag],
+    }))
+
+    renderPage ()
+
+    await screen.findByText ('Embed:上映中の投稿')
+
+    expect (within (tagSection ()).getByRole ('link', { name: '親タグ' }))
+      .toBeInTheDocument ()
+    expect (within (tagSection ()).getByRole ('link', { name: '別親タグ' }))
+      .toBeInTheDocument ()
+    expect (within (tagSection ()).getByRole ('link', { name: '子タグ' }))
+      .toBeInTheDocument ()
+    expect (within (tagSection ()).getAllByRole ('link', { name: '子タグ' }))
+      .toHaveLength (1)
+
+    fireEvent.click (screen.getByRole ('button', { name: '2 列 A 型' }))
+    fireEvent.click (screen.getAllByRole ('button', { name: '横並び' })[0])
+
+    await waitFor (() => {
+      expect (within (tagSection ()).getByRole ('link', { name: '親タグ' }))
+	.toBeInTheDocument ()
+      expect (within (tagSection ()).getByRole ('link', { name: '別親タグ' }))
+	.toBeInTheDocument ()
+      expect (within (tagSection ()).getByRole ('link', { name: '子タグ' }))
+	.toBeInTheDocument ()
+      expect (within (tagSection ()).getAllByRole ('link', { name: '子タグ' }))
+	.toHaveLength (1)
+    })
+  })
+
   it ('does not advance host post while video length is unknown', async () => {
     api.apiPut.mockImplementation ((path: string) => {
       switch (path)
diff --git a/frontend/src/pages/theatres/TheatreDetailPage.tsx b/frontend/src/pages/theatres/TheatreDetailPage.tsx
index f3f27aa..a52169f 100644
--- a/frontend/src/pages/theatres/TheatreDetailPage.tsx
+++ b/frontend/src/pages/theatres/TheatreDetailPage.tsx
@@ -102,10 +102,33 @@ const compareTagName = (a: Tag, b: Tag): number =>
     a.name === b.name ? 0 : (a.name < b.name ? -1 : 1)
 
 
-const tagsByCategory = (tags: Tag[]): Partial<Record<Category, Tag[]>> => {
-  const grouped: Partial<Record<Category, Tag[]>> = { }
+const flattenTags = (tags: Tag[]): Tag[] => {
+  const flattened: Tag[] = []
+  const seen = new Set<number> ()
+
+  const visit = (tag: Tag) => {
+    if (seen.has (tag.id))
+      return
+
+    seen.add (tag.id)
+    flattened.push (tag)
+
+    for (const child of tag.children ?? [])
+      visit (child)
+  }
 
   for (const tag of tags)
+    visit (tag)
+
+  return flattened
+}
+
+
+const tagsByCategory = (tags: Tag[]): Partial<Record<Category, Tag[]>> => {
+  const flattenedTags = flattenTags (tags)
+  const grouped: Partial<Record<Category, Tag[]>> = { }
+
+  for (const tag of flattenedTags)
     {
       grouped[tag.category] ??= []
       grouped[tag.category]!.push (tag)