diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 96f520c..000577b 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -392,7 +392,7 @@ class PostsController < ApplicationController
       tag = tags_by_id[tag_id]
       return nil unless tag
 
-      sections = PostTagSection.where(post_id: post.id, tag_id: tag.id)
+      sections = PostTagSection.where(post_id: post.id, tag_id:)
                  .as_json(only: [:begin_ms, :end_ms])
 
       if path.include?(tag_id)
@@ -419,7 +419,7 @@ class PostsController < ApplicationController
 
     params[:parent_post_ids].to_s.split.map { |token|
       id = Integer(token, exception: false)
-      raise ArgumentError, "親投稿 Id. が不正です: #{ token }" if id.nil? || id <= 0
+      raise ArgumentError, "親投稿 Id. が不正です: #{ token }" if !(id) || id <= 0
 
       id
     }.uniq
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 0015250..f65ce24 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -102,7 +102,7 @@ class Tag < ApplicationRecord
     tags = tag_names.map do |name|
       pf, cat = CATEGORY_PREFIXES.find { |p, _| name.downcase.start_with?(p) } || ['', nil]
 
-      name = TagName.canonicalise(name.sub(/\A#{ pf }/i, '')).first
+      name = name.sub(/\A#{ pf }/i, '')
 
       sections_by_tag = []
       while n = name.sub!(/^(\S*?)\[([0-9:.]*?)-([0-9:.]*?)\](\S*?)$/, '\1\4 \2 \3')
@@ -116,9 +116,14 @@ class Tag < ApplicationRecord
         sections_by_tag << [begin_ms, end_ms]
       end
 
+      name = TagName.canonicalise(name).first
+
       find_or_create_by_tag_name!(name, category: (cat || :general)).tap do |tag|
         tag.update!(category: cat) if cat && tag.category != cat
-        sections[tag.id] = sections_by_tag if sections_by_tag.present?
+        next if sections_by_tag.blank?
+
+        sections[tag.id] ||= []
+        sections[tag.id].concat(sections_by_tag)
       end
     end
 
diff --git a/backend/spec/factories/post_tag_sections.rb b/backend/spec/factories/post_tag_sections.rb
new file mode 100644
index 0000000..2560395
--- /dev/null
+++ b/backend/spec/factories/post_tag_sections.rb
@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :post_tag_section do
+    association :post
+    association :tag
+    begin_ms { 1_000 }
+    end_ms { 2_000 }
+  end
+end
diff --git a/backend/spec/factories/post_tags.rb b/backend/spec/factories/post_tags.rb
new file mode 100644
index 0000000..6956ca3
--- /dev/null
+++ b/backend/spec/factories/post_tags.rb
@@ -0,0 +1,6 @@
+FactoryBot.define do
+  factory :post_tag do
+    association :post
+    association :tag
+  end
+end
diff --git a/backend/spec/factories/posts.rb b/backend/spec/factories/posts.rb
new file mode 100644
index 0000000..9f46b2e
--- /dev/null
+++ b/backend/spec/factories/posts.rb
@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :post do
+    sequence(:url) { |n| "https://example.com/factory-post-#{ n }" }
+    title { 'factory post' }
+    thumbnail_base { nil }
+    uploaded_user { nil }
+  end
+end
diff --git a/frontend/src/components/PostEditForm.tsx b/frontend/src/components/PostEditForm.tsx
index 4655f01..2a2083e 100644
--- a/frontend/src/components/PostEditForm.tsx
+++ b/frontend/src/components/PostEditForm.tsx
@@ -12,21 +12,24 @@ import { msToTime } from '@/lib/utils'
 
 import type { FC, FormEvent } from 'react'
 
-import type { Post, Tag } from '@/types'
+import type { Post, TagWithSections } from '@/types'
 
 
-const tagsToStr = (tags: Tag[]): string => {
-  const result: Tag[] = []
+const tagsToStr = (tags: TagWithSections[]): string => {
+  const result: Omit<TagWithSections, 'children'>[] = []
 
-  const walk = (tag: Tag) => {
+  const walk = (tag: TagWithSections) => {
     const { children, ...rest } = tag
     result.push (rest)
-    children?.forEach (walk)
+    children.forEach (walk)
   }
 
   tags.filter (t => t.category !== 'nico').forEach (walk)
 
-  return [...(new Set (result.map (t => `${ t.name }${ t.sections.map (s => `[${ msToTime (s.beginMs) }-${ msToTime (s.endMs) }]`).join ('') }`)))].join (' ')
+  return [...(new Set (result.map (t =>
+      `${ t.name }${ t.sections
+		     .map (s => `[${ msToTime (s.beginMs) }-${ msToTime (s.endMs) }]`)
+		     .join ('') }`)))].join (' ')
 }
 
 
@@ -118,7 +121,7 @@ const PostEditForm: FC<Props> = ({ post, onSave }) => {
   }
 
   useEffect (() => {
-    setTags(tagsToStr (post.tags))
+    setTags (tagsToStr (post.tags))
   }, [post])
 
   return (
diff --git a/frontend/src/components/TagDetailSidebar.tsx b/frontend/src/components/TagDetailSidebar.tsx
index be95412..24dae59 100644
--- a/frontend/src/components/TagDetailSidebar.tsx
+++ b/frontend/src/components/TagDetailSidebar.tsx
@@ -26,13 +26,13 @@ import { dateString, originalCreatedAtString } from '@/lib/utils'
 import type { DragEndEvent } from '@dnd-kit/core'
 import type { FC, MutableRefObject, ReactNode } from 'react'
 
-import type { Category, Post, Tag } from '@/types'
+import type { Category, Post, TagWithSections } from '@/types'
 
-type TagByCategory = { [key in Category]: Tag[] }
+type TagByCategory = { [key in Category]: TagWithSections[] }
 
 
 const renderTagTree = (
-    tag:              Tag,
+    tag:              TagWithSections,
     nestLevel:        number,
     path:             string,
     suppressClickRef: MutableRefObject<boolean>,
@@ -63,7 +63,7 @@ const renderTagTree = (
 
 
 const isDescendant = (
-    root:     Tag,
+    root:     TagWithSections,
     targetId: number,
 ): boolean => {
   if (!(root.children))
@@ -84,8 +84,8 @@ const isDescendant = (
 const findTag = (
     byCat:  TagByCategory,
     id:     number,
-): Tag | undefined => {
-  const walk = (nodes: Tag[]): Tag | undefined => {
+): TagWithSections | undefined => {
+  const walk = (nodes: TagWithSections[]): TagWithSections | undefined => {
     for (const t of nodes)
       {
 	if (t.id === id)
@@ -167,7 +167,7 @@ const TagDetailSidebar: FC<Props> = ({ post, sp }) => {
       }
 
     for (const cat of Object.keys (tagsTmp) as (keyof typeof tagsTmp)[])
-      tagsTmp[cat].sort ((tagA: Tag, tagB: Tag) => tagA.name < tagB.name ? -1 : 1)
+      tagsTmp[cat].sort ((tagA: TagWithSections, tagB: TagWithSections) => tagA.name < tagB.name ? -1 : 1)
 
     return tagsTmp
   }, [post])
@@ -378,4 +378,4 @@ const TagDetailSidebar: FC<Props> = ({ post, sp }) => {
       </SidebarComponent>)
 }
 
-export default TagDetailSidebar
\ No newline at end of file
+export default TagDetailSidebar
diff --git a/frontend/src/lib/utils.ts b/frontend/src/lib/utils.ts
index 672ebd2..6493039 100644
--- a/frontend/src/lib/utils.ts
+++ b/frontend/src/lib/utils.ts
@@ -77,7 +77,7 @@ export const msToTime = (ms: number): string => {
   const totalS = Math.trunc (ms / 1_000)
   const s = String (totalS % 60)
   const min = String (Math.trunc (totalS / 60) % 60)
-  const h = String (Math.trunc (totalS / 3_600))
+  const h = Math.trunc (totalS / 3_600)
 
   return (h > 0
 	  ? `${ h }:${ min.padStart (2, '0') }:${ s.padStart (2, '0') }`
diff --git a/frontend/src/pages/tags/TagDetailPage.tsx b/frontend/src/pages/tags/TagDetailPage.tsx
index fc10727..7d6eb57 100644
--- a/frontend/src/pages/tags/TagDetailPage.tsx
+++ b/frontend/src/pages/tags/TagDetailPage.tsx
@@ -157,4 +157,4 @@ const TagDetailPage: FC = () => {
       </MainArea>)
 }
 
-export default TagDetailPage
\ No newline at end of file
+export default TagDetailPage
diff --git a/frontend/src/test/factories.ts b/frontend/src/test/factories.ts
index 2092619..929f4f1 100644
--- a/frontend/src/test/factories.ts
+++ b/frontend/src/test/factories.ts
@@ -1,6 +1,6 @@
-import type { Material, Post, Tag, User, WikiPage } from '@/types'
+import type { Material, Post, TagWithSections, User, WikiPage } from '@/types'
 
-export const buildTag = (overrides: Partial<Tag> = {}): Tag => ({
+export const buildTag = (overrides: Partial<TagWithSections> = {}): TagWithSections => ({
   id:             1,
   name:           'テストタグ',
   category:       'general',
@@ -13,6 +13,8 @@ export const buildTag = (overrides: Partial<Tag> = {}): Tag => ({
   materialId:     null,
   hasDeerjikists: false,
   matchedAlias:   null,
+  sections:       [],
+  children:       [],
   ...overrides,
 })