diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 74d720c..fda77ed 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -127,17 +127,20 @@ class PostsController < ApplicationController
     post = Post.new(title:, url:, thumbnail_base: nil, uploaded_user: current_user,
                     original_created_from:, original_created_before:)
     post.thumbnail.attach(thumbnail)
-    if post.save
-      post.resized_thumbnail!
+
+    ActiveRecord::Base.transaction do
+      post.save!
       tags = Tag.normalise_tags(tag_names)
       tags = Tag.expand_parent_tags(tags)
       sync_post_tags!(post, tags)
-
-      post.reload
-      render json: PostRepr.base(post), status: :created
-    else
-      render json: { errors: post.errors.full_messages }, status: :unprocessable_entity
+      post.resized_thumbnail!
+      PostVersionRecorder.record!(post:, event_type: :create, created_by_user: current_user)
     end
+
+    post.reload
+    render json: PostRepr.base(post), status: :created
+  rescue ActiveRecord::RecordInvalid
+    render json: { errors: post.errors.full_messages }, status: :unprocessable_entity
   rescue Tag::NicoTagNormalisationError
     head :bad_request
   end
@@ -166,19 +169,22 @@ class PostsController < ApplicationController
     original_created_before = params[:original_created_before]
 
     post = Post.find(params[:id].to_i)
-    if post.update(title:, original_created_from:, original_created_before:)
+
+    ActiveRecord::Base.transaction do
+      post.update!(title:, original_created_from:, original_created_before:)
       tags = post.tags.where(category: 'nico').to_a +
              Tag.normalise_tags(tag_names, with_tagme: false)
       tags = Tag.expand_parent_tags(tags)
       sync_post_tags!(post, tags)
-
-      post.reload
-      json = post.as_json
-      json['tags'] = build_tag_tree_for(post.tags)
-      render json:, status: :ok
-    else
-      render json: post.errors, status: :unprocessable_entity
+      PostVersionRecorder.record!(post:, event_type: :update, created_by_user: current_user)
     end
+
+    post.reload
+    json = post.as_json
+    json['tags'] = build_tag_tree_for(post.tags)
+    render json:, status: :ok
+  rescue ActiveRecord::RecordInvalid
+    render json: post.errors, status: :unprocessable_entity
   rescue Tag::NicoTagNormalisationError
     head :bad_request
   end
diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index c898615..901b1e3 100644
--- a/backend/app/models/post.rb
+++ b/backend/app/models/post.rb
@@ -11,6 +11,7 @@ class Post < ApplicationRecord
 
   has_many :user_post_views, dependent: :delete_all
   has_many :post_similarities, dependent: :delete_all
+  has_many :post_versions
 
   has_one_attached :thumbnail
 
@@ -30,6 +31,8 @@ class Post < ApplicationRecord
     super(options).merge(thumbnail: nil)
   end
 
+  def snapshot_tag_names = tags.joins(:tag_name).order('tag_names.name').pluck('tag_names.name')
+
   def related limit: nil
     ids = post_similarities.order(cos: :desc)
     ids = ids.limit(limit) if limit
diff --git a/backend/app/models/post_version.rb b/backend/app/models/post_version.rb
new file mode 100644
index 0000000..a11ee89
--- /dev/null
+++ b/backend/app/models/post_version.rb
@@ -0,0 +1,35 @@
+class PostVersion < ApplicationRecord
+  before_update do
+    raise ActiveRecord::ReadOnlyRecord, '版は更新できません．'
+  end
+
+  before_destroy do
+    raise ActiveRecord::ReadOnlyRecord, '版は削除できません．'
+  end
+
+  belongs_to :post
+  belongs_to :parent, class_name: 'Post', optional: true
+  belongs_to :created_by_user, class_name: 'User', optional: true
+
+  enum :event_type, create: 'create', update: 'update', discard: 'discard', restore: 'restore'
+
+  validates :version_no, presence: true, numerically: { only_integer: true, greater_than: 0 }
+  validates :event_type, presence: true, inclusion: { in: event_types.keys }
+  validates :url, presence: true
+
+  validate :validate_original_created_range
+
+  scope :chronological, -> { order(:version_no, :id) }
+
+  private
+
+  def validate_original_created_range
+    f = original_created_from
+    b = original_created_before
+    return if f.blank? || b.blank?
+
+    if f >= b
+      errors.add :original_created_before, 'オリジナルの作成日時の順番がをかしぃです．'
+    end
+  end
+end
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 64e0b9b..d4edd8d 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -1,3 +1,6 @@
+require 'set'
+
+
 class Tag < ApplicationRecord
   include MyDiscard
 
@@ -150,6 +153,8 @@ class Tag < ApplicationRecord
   def self.merge_tags! target_tag, source_tags
     target_tag => Tag
 
+    affected_post_ids = Set.new
+
     Tag.transaction do
       Array(source_tags).compact.uniq.each do |source_tag|
         source_tag => Tag
@@ -158,6 +163,7 @@ class Tag < ApplicationRecord
 
         source_tag.post_tags.kept.find_each do |source_pt|
           post_id = source_pt.post_id
+          affected_post_ids << post_id
           source_pt.discard_by!(nil)
           unless PostTag.kept.exists?(post_id:, tag: target_tag)
             PostTag.create!(post_id:, tag: target_tag)
@@ -180,6 +186,10 @@ class Tag < ApplicationRecord
         end
       end
 
+      Post.where(id: affected_post_ids.to_a).find_each do |post|
+        PostVersionRecorder.record!(post:, event_type: :update, created_by_user: nil)
+      end
+
       # 投稿件数を再集計
       target_tag.update_columns(post_count: PostTag.kept.where(tag: target_tag).count)
     end
diff --git a/backend/app/services/post_version_recorder.rb b/backend/app/services/post_version_recorder.rb
new file mode 100644
index 0000000..20f5c1e
--- /dev/null
+++ b/backend/app/services/post_version_recorder.rb
@@ -0,0 +1,57 @@
+class PostVersionRecorder
+  def self.record! post:, event_type:, created_by_user:
+    new(post:, event_type:, created_by_user:).record!
+  end
+
+  def initialize post:, event_type:, created_user:
+    @post = post
+    @event_type = event_type
+    @created_by_user = created_by_user
+  end
+
+  def record!
+    @post.with_lock do
+      latest = @post.post_versions.order(version_no: :desc).first
+      attrs = snapshot_attributes
+
+      return latest if @event_type == :update && latest && same_snapshot?(latest, attrs)
+
+      PostVersion.create!(
+          post: @post,
+          version_no: (latest&.version_no || 0) + 1,
+          event_type: @event_type,
+          title: attrs[:title],
+          url: attrs[:url],
+          thumbnail_base: attrs[:thumbnail_base],
+          tags: attrs[:url],
+          parent: attrs[:parent],
+          original_created_from: attrs[:original_created_from],
+          original_created_before: attrs[:original_created_before],
+          created_at: Time.current,
+          created_by_user: @created_by_user)
+    end
+  end
+
+  private
+
+  def snapshot_attributes
+    { title: @post.title,
+      url: @post.url,
+      thumbnail_base: @post.thumbnail_base,
+      tags: @post.snapshot_tag_names.join(' '),
+      parent: @post.parent,
+      original_created_from: @post.original_created_from,
+      original_created_before: @post.original_created_before }
+  end
+
+  def same_snapshot? version, attrs
+    true &&
+        version.title == attrs[:title] &&
+        version.url == attrs[:url] &&
+        version.thumbnail_base == attrs[:thumbnail_base] &&
+        version.tags == attrs[:tags] &&
+        version.parent_id == attrs[:parent]&.id &&
+        version.original_created_from == attrs[:original_created_from] &&
+        version.original_created_before == attrs[:original_created_before]
+  end
+end
diff --git a/backend/db/migrate/20260409123700_create_post_versions.rb b/backend/db/migrate/20260409123700_create_post_versions.rb
new file mode 100644
index 0000000..f1c884e
--- /dev/null
+++ b/backend/db/migrate/20260409123700_create_post_versions.rb
@@ -0,0 +1,80 @@
+class CreatePostVersions < ActiveRecord::Migration[8.0]
+  def change
+    create_table :post_versions do |t|
+      t.references :post, null: false, foreign_key: true
+      t.integer :version_no, null: false
+      t.string :event_type, null: false
+      t.string :title
+      t.string :url, limit: 768, null: false
+      t.string :thumbnail_base, limit: 2000
+      t.text :tags, null: false
+      t.references :parent, foreign_key: { to_table: :posts }
+      t.datetime :original_created_from
+      t.datetime :original_created_before
+      t.datetime :created_at, null: false
+      t.references :created_by_user, foreign_key: { to_table: :users }
+
+      t.index [:post_id, :version_no], unique: true
+      t.check_constraint 'version_no > 0'
+      t.check_constraint "event_type IN ('create', 'update', 'discard', 'restore')"
+    end
+
+    reversible do |dir|
+      dir.up do
+        execute <<~SQL
+          INSERT INTO
+            post_versions(
+              post_id
+            , version_no
+            , event_type
+            , title
+            , url
+            , thumbnail_base
+            , tags
+            , parent_id
+            , original_created_from
+            , original_created_before
+            , created_at
+            , created_by_user_id)
+          SELECT
+            posts.id
+          , 1
+          , 'create'
+          , posts.title
+          , posts.url
+          , posts.thumbnail_base
+          , COALESCE(tag_snapshots.tags, '')
+          , posts.parent_id
+          , posts.original_created_from
+          , posts.original_created_before
+          , posts.created_at
+          , posts.uploaded_user_id
+          FROM
+            posts
+            LEFT JOIN
+              (
+                SELECT
+                  post_tags.post_id
+                , GROUP_CONCAT(tag_names.name ORDER BY tag_names.name SEPARATOR ' ') AS tags
+                FROM
+                  post_tags
+                  INNER JOIN
+                    tags
+                    ON
+                      tags.id = post_tags.tag_id
+                  INNER JOIN
+                    tag_names
+                    ON
+                      tag_names.id = tags.tag_name_id
+                WHERE
+                  post_tags.discarded_at IS NULL
+                GROUP BY
+                  post_tags.post_id
+                ) tag_snapshots
+              ON
+                tag_snapshots.post_id = posts.id
+        SQL
+      end
+    end
+  end
+end
diff --git a/backend/lib/tasks/sync_nico.rake b/backend/lib/tasks/sync_nico.rake
index 09be474..da396a0 100644
--- a/backend/lib/tasks/sync_nico.rake
+++ b/backend/lib/tasks/sync_nico.rake
@@ -61,6 +61,9 @@ namespace :nico do
       original_created_from = original_created_at&.change(sec: 0)
       original_created_before = original_created_from&.+(1.minute)
 
+      post_created = false
+      post_changed = false
+
       if post
         attrs = { title:, original_created_from:, original_created_before: }
 
@@ -76,11 +79,13 @@ namespace :nico do
         end
 
         post.assign_attributes(attrs)
-        if post.changed?
+        post_changed = post.changed?
+        if post_changed
           post.save!
           post.resized_thumbnail! if post.thumbnail.attached?
         end
       else
+        post_created = true
         url = "https://www.nicovideo.jp/watch/#{ code }"
         thumbnail_base = fetch_thumbnail.(url) rescue nil
         post = Post.new(title:, url:, thumbnail_base:, uploaded_user: nil,
@@ -140,6 +145,12 @@ namespace :nico do
       desired_all_tag_ids.uniq!
 
       sync_post_tags!(post, desired_all_tag_ids, current_tag_ids: kept_tag_ids)
+
+      if post_created
+        PostVersionRecorder.record!(post:, event_type: :create, created_by_user: nil)
+      elsif post_changed || kept_tag_ids != desired_all_tag_ids.to_set
+        PostVersionRecorder.record!(post:, event_type: :update, created_by_user: nil)
+      end
     end
   end
 end