diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 272db8b..e0768de 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -44,7 +44,7 @@ class PostsController < ApplicationController
         filtered_posts
         .joins("LEFT JOIN (#{ pt_max_sql }) pt_max ON pt_max.post_id = posts.id")
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
-        .preload(tags: [:materials, { tag_name: :wiki_page }])
+        .preload(tags: [:deerjikists, :materials, { tag_name: :wiki_page }])
         .with_attached_thumbnail
 
     q = q.where('posts.url LIKE ?', "%#{ url }%") if url
@@ -95,7 +95,7 @@ class PostsController < ApplicationController
   end
 
   def random
-    post = filtered_posts.preload(tags: [:materials, { tag_name: :wiki_page }])
+    post = filtered_posts.preload(tags: [:deerjikists, :materials, { tag_name: :wiki_page }])
                          .order('RAND()')
                          .first
     return head :not_found unless post
@@ -104,7 +104,7 @@ class PostsController < ApplicationController
   end
 
   def show
-    post = Post.includes(tags: [:materials, { tag_name: :wiki_page }]).find_by(id: params[:id])
+    post = Post.includes(tags: [:deerjikists, :materials, { tag_name: :wiki_page }]).find_by(id: params[:id])
     return head :not_found unless post
 
     render json: PostRepr.base(post, current_user)
@@ -173,8 +173,12 @@ class PostsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
-    base_version_no = parse_base_version_no
     force = truthy_param?(params[:force])
+    merge = truthy_param?(params[:merge])
+    return head :bad_request if force && merge
+
+    base_version_no = nil
+    base_version_no = parse_base_version_no unless force
 
     title = params[:title].presence
     tag_names = params[:tags].to_s.split
@@ -186,12 +190,17 @@ class PostsController < ApplicationController
     conflict_json = nil
 
     ApplicationRecord.transaction do
-      post = Post.find(params[:id].to_i)
+      post = Post.lock.find(params[:id].to_i)
 
-      base_version = post.post_versions.find_by!(version_no: base_version_no)
+      base_version = nil
+      base_snapshot = nil
+      current_snapshot = nil
+      unless force
+        base_version = post.post_versions.find_by!(version_no: base_version_no)
 
-      base_snapshot = post_snapshot_from_version(base_version)
-      current_snapshot = post_snapshot_from_record(post)
+        base_snapshot = post_snapshot_from_version(base_version)
+        current_snapshot = post_snapshot_from_record(post)
+      end
       incoming_snapshot = post_incoming_snapshot(post,
                                                  title:,
                                                  original_created_from:,
@@ -199,29 +208,28 @@ class PostsController < ApplicationController
                                                  tag_names:,
                                                  parent_post_ids:)
 
-      if !(force) && post.version_no != base_version_no
-        conflict_json = post_conflict_json(post:,
-                                           base_version_no:,
-                                           base_snapshot:,
-                                           current_snapshot:,
-                                           incoming_snapshot:)
-        raise ActiveRecord::Rollback
-      end
-
-      PostVersionRecorder.ensure_snapshot!(post, created_by_user: current_user)
-
-      post.update!(title:, original_created_from:, original_created_before:)
-
-      normalised_tags = Tag.normalise_tags!(tag_names, with_tagme: false)
-      TagVersioning.record_tag_snapshots!(normalised_tags, created_by_user: current_user)
-
-      tags = post.tags.nico.to_a + normalised_tags
-      tags = Tag.expand_parent_tags(tags)
-      sync_post_tags!(post, tags)
+      snapshot_to_apply =
+          if post.version_no == base_version_no || force
+            incoming_snapshot
+          else
+            changes = post_snapshot_changes(base_snapshot, current_snapshot, incoming_snapshot)
+            conflicts = changes.select { |change| change[:conflict] }
 
-      sync_parent_posts!(post, parent_post_ids)
+            if merge && conflicts.empty?
+              merge_post_snapshots(base_snapshot, current_snapshot, incoming_snapshot)
+            else
+              conflict_json = post_conflict_json(post:,
+                                                 base_version_no:,
+                                                 base_snapshot:,
+                                                 current_snapshot:,
+                                                 incoming_snapshot:,
+                                                 changes:,
+                                                 conflicts:)
+              raise ActiveRecord::Rollback
+            end
+          end
 
-      PostVersionRecorder.record!(post:, event_type: :update, created_by_user: current_user)
+      apply_post_snapshot!(post, snapshot_to_apply)
     end
 
     return render json: conflict_json, status: :conflict if conflict_json
@@ -253,7 +261,7 @@ class PostsController < ApplicationController
     pts = pts.where(post_id: id) if id.present?
     pts = pts.where(tag_id:) if tag_id.present?
     pts = pts.includes(:post, :created_user, :deleted_user,
-                       tag: [:materials, { tag_name: :wiki_page }])
+                       tag: [:deerjikists, :materials, { tag_name: :wiki_page }])
 
     events = []
     pts.each do |pt|
@@ -446,11 +454,11 @@ class PostsController < ApplicationController
     { title: version.title,
       original_created_from: snapshot_time(version.original_created_from),
       original_created_before: snapshot_time(version.original_created_before),
-      tag_names: version.tags.to_s.split.sort,
+      tag_names: version.tags.to_s.split.filter { !(_1.start_with?('nico:')) }.sort,
       parent_post_ids: snapshot_parent_post_ids_from_version(version) }
   end
 
-  def post_snapshot_form_record post
+  def post_snapshot_from_record post
     { title: post.title,
       original_created_from: snapshot_time(post.original_created_from),
       original_created_before: snapshot_time(post.original_created_before),
@@ -460,7 +468,7 @@ class PostsController < ApplicationController
 
   def post_incoming_snapshot post, title:, original_created_from:, original_created_before:,
                                    tag_names:, parent_post_ids:
-    { title:
+    { title:,
       original_created_from: snapshot_time(original_created_from),
       original_created_before: snapshot_time(original_created_before),
       tag_names: incoming_tag_names_for_snapshot(post, tag_names),
@@ -488,17 +496,16 @@ class PostsController < ApplicationController
 
   def incoming_tag_names_for_snapshot post, raw_tag_names
     manual_names = normalised_manual_tag_names_for_snapshot(raw_tag_names)
-    nico_names = post.tags.nico.joins(:tag_name).pluck('tag_names.name')
 
     existing_tags =
         Tag
         .joins(:tag_name)
-        .where(tag_names: { name: manual_names + nico_names })
+        .where(tag_names: { name: manual_names })
         .to_a
 
     expanded_names = Tag.expand_parent_tags(existing_tags).map(&:name)
 
-    (manual_names + nico_names + expanded_names).uniq.sort
+    (manual_names + expanded_names).uniq.sort
   end
 
   def normalised_manual_tag_names_for_snapshot raw_tag_names
@@ -528,10 +535,7 @@ class PostsController < ApplicationController
   end
 
   def post_conflict_json post:, base_version_no:, base_snapshot:,
-                         current_snapshot:, incoming_snapshot:
-    changes = post_snapshot_changes(base_snapshot, current_snapshot, incoming_snapshot)
-    conflicts = changes.select { |change| change[:conflict] }
-
+                         current_snapshot:, incoming_snapshot:, changes:, conflicts:
     { error: 'conflict',
       message: '競合が発生しました．',
       post_id: post.id,
@@ -548,9 +552,9 @@ class PostsController < ApplicationController
   def post_snapshot_changes base_snapshot, current_snapshot, incoming_snapshot
     [scalar_snapshot_change(:title, 'タイトル',
                             base_snapshot, current_snapshot, incoming_snapshot),
-     scalar_snapshot_change(:original_created_from, '元コンテンツ作成日時（開始）',
+     scalar_snapshot_change(:original_created_from, 'オリジナルの作成日時（以降）',
                             base_snapshot, current_snapshot, incoming_snapshot),
-     scalar_snapshot_change(:original_created_before, '元コンテンツ作成日時（終了）',
+     scalar_snapshot_change(:original_created_before, 'オリジナルの作成日時（より前）',
                             base_snapshot, current_snapshot, incoming_snapshot),
      set_snapshot_change(:tag_names, 'タグ',
                          base_snapshot, current_snapshot, incoming_snapshot),
@@ -606,4 +610,37 @@ class PostsController < ApplicationController
                              added_by_me:, removed_by_me:
     (added_by_current & removed_by_me).present? || (removed_by_current & added_by_me).present?
   end
+
+  def apply_post_snapshot! post, snapshot
+    PostVersionRecorder.ensure_snapshot!(post, created_by_user: current_user)
+
+    post.update!(title: snapshot[:title],
+                 original_created_from: snapshot[:original_created_from],
+                 original_created_before: snapshot[:original_created_before])
+
+    tags = Tag.normalise_tags!(snapshot[:tag_names], with_tagme: false)
+    TagVersioning.record_tag_snapshots!(tags, created_by_user: current_user)
+
+    tags = Tag.expand_parent_tags(tags)
+    sync_post_tags!(post, tags)
+
+    sync_parent_posts!(post, snapshot[:parent_post_ids])
+
+    PostVersionRecorder.record!(post:, event_type: :update, created_by_user: current_user)
+  end
+
+  def merge_post_snapshots base_snapshot, current_snapshot, incoming_snapshot
+    [:title, :original_created_from, :original_created_before, :tag_names, :parent_post_ids].map {
+      [_1, merge_scaler_snapshot_value(base_snapshot[_1],
+                                       current_snapshot[_1],
+                                       incoming_snapshot[_1])]
+    }.to_h
+  end
+
+  def merge_scaler_snapshot_value base, current, mine
+    return mine if current == base
+    return current if mine == base || current == mine
+
+    raise ArgumentError, '競合してゐる項目はマージできません．'
+  end
 end
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 402e4fe..fe4e1d7 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -136,6 +136,7 @@ class Tag < ApplicationRecord
     tn = tn.canonical if tn.canonical_id?
 
     Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) do |t|
+      t.version_no = TagVersion.where(tag_id: t.id).order(version_no: :desc).first || 1
       t.category = category
     end
   rescue ActiveRecord::RecordNotUnique
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index f52209d..da11ba2 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -8,6 +8,7 @@ import { BrowserRouter,
 
 import RouteBlockerOverlay from '@/components/RouteBlockerOverlay'
 import TopNav from '@/components/TopNav'
+import DialogueProvider from '@/components/dialogues/DialogueProvider'
 import { Toaster } from '@/components/ui/toaster'
 import { apiPost, isApiError } from '@/lib/api'
 import DeerjikistDetailPage from '@/pages/deerjikists/DeerjikistDetailPage'
@@ -138,17 +139,21 @@ export default (() => {
   return (
       <>
 	<RouteBlockerOverlay/>
+
 	<BrowserRouter>
-	  <LayoutGroup>
-	    <motion.div
-		layout="position"
-		transition={{ layout: { duration: .2, ease: 'easeOut' } }}
-		className="flex flex-col h-dvh w-full overflow-y-hidden">
-	      <TopNav user={user}/>
-	      <RouteTransitionWrapper user={user} setUser={setUser}/>
-	    </motion.div>
-	  </LayoutGroup>
-	  <Toaster/>
+	  <DialogueProvider>
+	    <LayoutGroup>
+	      <motion.div
+		  layout="position"
+		  transition={{ layout: { duration: .2, ease: 'easeOut' } }}
+		  className="flex flex-col h-dvh w-full overflow-y-hidden">
+		<TopNav user={user}/>
+		<RouteTransitionWrapper user={user} setUser={setUser}/>
+	      </motion.div>
+	    </LayoutGroup>
+
+	    <Toaster/>
+	  </DialogueProvider>
 	</BrowserRouter>
       </>)
 }) satisfies FC
diff --git a/frontend/src/components/PostEditForm.tsx b/frontend/src/components/PostEditForm.tsx
index 24d2dcc..ed1fa08 100644
--- a/frontend/src/components/PostEditForm.tsx
+++ b/frontend/src/components/PostEditForm.tsx
@@ -3,6 +3,7 @@ import { useEffect, useState } from 'react'
 import PostFormTagsArea from '@/components/PostFormTagsArea'
 import PostOriginalCreatedTimeField from '@/components/PostOriginalCreatedTimeField'
 import Label from '@/components/common/Label'
+import { useDialogue } from '@/components/dialogues/DialogueProvider'
 import { Button } from '@/components/ui/button'
 import { toast } from '@/components/ui/use-toast'
 import { updatePost } from '@/lib/posts'
@@ -41,29 +42,68 @@ export default (({ post, onSave }: Props) => {
   const [tags, setTags] = useState<string> ('')
   const [title, setTitle] = useState (post.title)
 
-  const handleSubmit = async () => {
+  const dialogue = useDialogue ()
+
+  const update = async (...args: Parameters<typeof updatePost>) => {
     try
     {
-      const data =
-	  await updatePost ({ id: post.id, versionNo: post.versionNo + 1,
-			      title, tags, parentPostIds,
-			      originalCreatedFrom, originalCreatedBefore })
+      const data = await updatePost (...args)
       onSave ({ ...post,
+		versionNo:              data.versionNo,
 		title:                  data.title,
 		tags:                   data.tags,
 		parentPosts:            data.parentPosts,
 		childPosts:             data.childPosts,
 		siblingPosts:           data.siblingPosts,
-	        originalCreatedFrom:    data.originalCreatedFrom,
+		originalCreatedFrom:    data.originalCreatedFrom,
 		originalCreatedBefore:  data.originalCreatedBefore } as Post)
       toast ({ description: '更新しました．' })
     }
-    catch
+    catch (e)
     {
-      toast ({ description: '更新はできなかったよ……' })
+      if (e.response.status !== 409)
+	{
+	  toast ({ description: '更新はできなかったよ……' })
+	  return
+	}
+
+      const action = await dialogue.choice ({
+	  title: '競合が発生しました．',
+	  description: (
+	      <div>
+		<p>ほかの耕作員が先に更新してゐます．</p>
+		<p>現在の変更をどう扱ひますか?</p>
+	      </div>),
+	  choices: [{ value: 'merge', label: '差分をマージ' },
+		    { value: 'overwrite', label: '強制上書き', variant: 'danger' }] })
+
+      if (action === 'merge')
+	{
+	  // TODO: 差分 UI
+	  await update ({ id: post.id, title, tags, parentPostIds,
+			  originalCreatedFrom, originalCreatedBefore },
+			{ baseVersionNo: post.versionNo, merge: true })
+	  return
+	}
+
+      if (action === 'overwrite')
+	{
+	  await update ({ id: post.id, title, tags, parentPostIds,
+			  originalCreatedFrom, originalCreatedBefore },
+			{ baseVersionNo: post.versionNo, force: true })
+	  return
+	}
     }
   }
 
+  const handleSubmit = async e => {
+    e.preventDefault ()
+
+    await update ({ id: post.id, title, tags, parentPostIds,
+		    originalCreatedFrom, originalCreatedBefore },
+		  { baseVersionNo: post.versionNo })
+  }
+
   useEffect (() => {
     setTags(tagsToStr (post.tags))
   }, [post])
diff --git a/frontend/src/components/dialogues/DialogueProvider.tsx b/frontend/src/components/dialogues/DialogueProvider.tsx
new file mode 100644
index 0000000..8cb0990
--- /dev/null
+++ b/frontend/src/components/dialogues/DialogueProvider.tsx
@@ -0,0 +1,184 @@
+import { createContext, useCallback, useContext, useMemo, useState } from 'react'
+
+import { Button } from '@/components/ui/button'
+import { Dialog,
+	 DialogContent,
+	 DialogDescription,
+	 DialogFooter,
+	 DialogTitle } from '@/components/ui/dialog'
+
+import type { FC, ReactNode } from 'react'
+
+type DialogueVariant = 'default' | 'danger'
+
+type ConfirmOptions = { title:        string
+			description?: ReactNode
+			confirmText?: string
+			cancelText?:  string
+			variant?:     DialogueVariant }
+
+type AlertOptions = { title:        string
+		      description?: ReactNode
+		      okText?:      string }
+
+type Choice<T extends string> = { value:    T
+				  label:    string
+				  variant?: DialogueVariant }
+
+type ChoiceOptions<T extends string> = { title:         string
+					 description?:  ReactNode
+					 choices:       Choice<T>[]
+					 cancelText?:   string }
+
+type DialogueRequest =
+    | { id:       number
+	kind:     'confirm'
+	options:  ConfirmOptions
+	resolve:  (value: boolean) => void }
+    | { id:       number
+	kind:     'alert'
+	options:  AlertOptions
+	resolve:  () => void }
+    | { id:       number
+	kind:     'choice'
+	options:  ChoiceOptions<string>
+	resolve:  (value: string | null) => void }
+
+type DialogueAPI =
+    { confirm:  (options: ConfirmOptions) => Promise<boolean>
+      alert:    (options: AlertOptions) => Promise<void>
+      choice:   <T extends string> (options: ChoiceOptions<T>) => Promise<T | null> }
+
+const DialogueContext = createContext<DialogueAPI | null> (null)
+
+let nextDialogueId = 1
+
+type Props = { children: ReactNode }
+
+
+export default (({ children }: Props) => {
+  const [queue, setQueue] = useState<DialogueRequest[]> ([])
+
+  const push = useCallback ((request: Omit<DialogueRequest, 'id'>) => {
+    const id = nextDialogueId
+    ++nextDialogueId
+
+    setQueue (q => [...q, { ...request, id } as DialogueRequest])
+  }, [])
+
+  const closeActive = useCallback ((result?: unknown) => {
+    setQueue (q => {
+      const [active, ...rest] = q
+
+      if (!(active))
+	return rest
+
+      switch (active.kind)
+      {
+      case 'confirm':
+	active.resolve (Boolean (result))
+	break
+
+      case 'alert':
+	active.resolve ()
+	break
+
+      case 'choice':
+	active.resolve ((result ?? null) as string | null)
+	break
+      }
+
+      return rest
+    })
+  }, [])
+
+  const api = useMemo<DialogueAPI> (() => ({
+      confirm: options => new Promise<boolean> (resolve => {
+	push ({ kind: 'confirm', options, resolve })
+      }),
+      alert: options => new Promise<void> (resolve => {
+	push ({ kind: 'alert', options, resolve })
+      }),
+      choice: options => new Promise (resolve => {
+	push ({ kind: 'choice',
+		options: options as ChoiceOptions<string>,
+		resolve: resolve as (value: string | null) => void })
+      }) }), [push])
+
+  const active = queue[0]
+
+  return (
+      <DialogueContext.Provider value={api}>
+	{children}
+
+	<Dialog
+	    open={Boolean (active)}
+	    onOpenChange={open => {
+	      if (!(open))
+		closeActive (active?.kind !== 'confirm' && null)
+	    }}>
+	  {active && (
+	      <DialogContent>
+		<DialogTitle>{active.options.title}</DialogTitle>
+
+		{active.options.description && (
+		    <DialogDescription asChild>
+		      <div>{active.options.description}</div>
+		    </DialogDescription>)}
+
+		<DialogFooter>
+		  {active.kind === 'confirm' && (
+		      <>
+			<Button
+			    variant="outline"
+			    onClick={() => closeActive (false)}>
+			  {active.options.cancelText ?? '取消'}
+			</Button>
+
+			<Button
+			    variant={(active.options.variant === 'danger')
+				     ? 'destructive'
+				     : 'default'}
+			    onClick={() => closeActive (true)}>
+			  {active.options.confirmText ?? '確定'}
+			</Button>
+		      </>)}
+
+		  {active.kind === 'alert' && (
+		      <Button onClick={() => closeActive ()}>
+			{active.options.okText ?? '確定'}
+		      </Button>)}
+
+		  {active.kind === 'choice' && (
+		      <>
+			<Button
+			    variant="outline"
+			    onClick={() => closeActive (null)}>
+			  {active.options.cancelText ?? '取消'}
+			</Button>
+
+			{active.options.choices.map (choice => (
+			    <Button
+				key={choice.value}
+				variant={(choice.variant === 'danger')
+					 ? 'destructive'
+					 : 'default'}
+				onClick={() => closeActive (choice.value)}>
+			      {choice.label}
+			    </Button>))}
+		      </>)}
+		</DialogFooter>
+	      </DialogContent>)}
+	</Dialog>
+      </DialogueContext.Provider>)
+}) satisfies FC<Props>
+
+
+export const useDialogue = () => {
+  const dialogue = useContext (DialogueContext)
+
+  if (!(dialogue))
+    throw new Error ('useDialogue must be used inside DialogueProvider')
+
+  return dialogue
+}
diff --git a/frontend/src/lib/posts.ts b/frontend/src/lib/posts.ts
index 4197148..a1febff 100644
--- a/frontend/src/lib/posts.ts
+++ b/frontend/src/lib/posts.ts
@@ -44,21 +44,26 @@ export const fetchPostChanges = async (
 
 export const updatePost = async (
     post: { id:                     number
-	    versionNo:              number
 	    title:                  string | null
 	    tags:                   string
 	    parentPostIds:          string
 	    originalCreatedFrom:    string | null
 	    originalCreatedBefore:  string | null },
+    { baseVersionNo, force, merge }: {
+	baseVersionNo?: number
+	force?:         boolean
+	merge?:         boolean }
 ) =>
     await apiPut<Post> (
 	`/posts/${ post.id }`,
-	{ version_no: post.versionNo,
-	  title: post.title,
+	{ title: post.title,
 	  tags: post.tags,
 	  parent_post_ids: post.parentPostIds,
 	  original_created_from: post.originalCreatedFrom,
-	  original_created_before: post.originalCreatedBefore })
+	  original_created_before: post.originalCreatedBefore },
+	{ params: { ...(baseVersionNo && { base_version_no: String (baseVersionNo) }),
+	            force: force ? '1' : '0',
+		    merge: merge ? '1' : '0' } })
 
 
 export const toggleViewedFlg = async (id: string, viewed: boolean): Promise<void> => {
diff --git a/frontend/src/pages/posts/PostHistoryPage.tsx b/frontend/src/pages/posts/PostHistoryPage.tsx
index 0545b93..8e6f21e 100644
--- a/frontend/src/pages/posts/PostHistoryPage.tsx
+++ b/frontend/src/pages/posts/PostHistoryPage.tsx
@@ -73,7 +73,6 @@ export default (() => {
     try
     {
       const id = change.postId
-      const versionNo = change.latestVersionNo + 1
       const title = change.title.current
       const tags =
 	  change.tags
@@ -88,8 +87,9 @@ export default (() => {
 	  .join (' ')
       const originalCreatedFrom = change.originalCreatedFrom.current
       const originalCreatedBefore = change.originalCreatedBefore.current
-      await updatePost ({ id, versionNo, title, tags, parentPostIds,
-			  originalCreatedFrom, originalCreatedBefore })
+      await updatePost ({ id, title, tags, parentPostIds,
+			  originalCreatedFrom, originalCreatedBefore },
+		        { force: true })
 
       qc.invalidateQueries ({ queryKey: postsKeys.root })
       qc.invalidateQueries ({ queryKey: tagsKeys.root })