diff --git a/app/controllers/documents_controller.rb b/app/controllers/documents_controller.rb
index 39e9d69d..6005754b 100644
--- a/app/controllers/documents_controller.rb
+++ b/app/controllers/documents_controller.rb
@@ -1,6 +1,7 @@
 # coding: utf-8
 class DocumentsController < ApplicationController
   before_action :set_document, only: %i[ show edit update destroy ]
+  before_action :get_form_data, only: %i[ new edit ]
   protect_from_forgery :except => [:api_markdown]
 
   # GET /documents or /documents.json
@@ -26,9 +27,6 @@ def show
   # GET /documents/new
   def new
     @document = Document.new
-    @users = User.where(active: true)
-    @projects = Project.all
-    @tags = Tag.all
     # TODO: バージョン7.1以降では datetime_field に include_seconds オプションが導入されるため，以下の秒を0にする記述は不要になる
     @document.start_at = DateTime.current.change(sec: 0)
     @document.end_at = DateTime.current.change(sec: 0)
@@ -41,9 +39,6 @@ def new
 
   # GET /documents/1/edit
   def edit
-    @users = User.where(active: true)
-    @projects = Project.all
-    @tags = Tag.all
   end
 
   # POST /documents or /documents.json
@@ -51,13 +46,15 @@ def create
     @document = current_user.documents.build(document_params)
     parse_tag_names(params[:tag_names]) if params[:tag_names]
 
-    if @document.save #XXX: save! => save
+    if @document.save
       respond_to do |format|
         format.html { redirect_to @document, notice: "文書を追加しました" }
         format.json { render :show, status: :created, location: @document }
       end
     else
       respond_to do |format|
+        flash.now[:danger] = '文書の作成に失敗しました．'
+        get_form_data
         format.html { render :new, status: :unprocessable_entity }
         format.json { render json: @document.errors, status: :unprocessable_entity }
       end
@@ -66,15 +63,28 @@ def create
 
   # PATCH/PUT /documents/1 or /documents/1.json
   def update
-    parse_tag_names(params[:tag_names]) if params[:tag_names]
-    if @document.update(document_params)
-      flash[:success] = "文書を更新しました"
-      redirect_to @document
-    else
-      respond_to do |format|
-        format.html { render :edit, status: :unprocessable_entity }
-        format.json { render json: @document.errors, status: :unprocessable_entity }
-      end
+    @document.transaction do
+      parse_tag_names(params[:tag_names]) if params[:tag_names]
+      @document.update!(document_params)
+    end
+
+    respond_to do |format|
+      format.html { redirect_to @document, notice: '文書を更新しました' }
+      format.json { render :show, status: :ok }
+    end
+  rescue ActiveRecord::StaleObjectError
+    flash.now[:danger] = 'この文書は他のユーザーによって更新されました．'
+    get_form_data
+    respond_to do |format|
+      format.html { render :edit, status: :conflict }
+      format.json { render json: { error: flash[:error] }, status: :conflict }
+    end
+  rescue
+    flash.now[:danger] = '文書の更新に失敗しました．'
+    get_form_data
+    respond_to do |format|
+      format.html { render :edit, status: :unprocessable_entity }
+      format.json { render json: @document.errors, status: :unprocessable_entity }
     end
   end
 
@@ -101,7 +111,7 @@ def set_document
 
   # Only allow a list of trusted parameters through.
   def document_params
-    params.require(:document).permit(:creator_id, :content, :description, :project_id, :start_at, :end_at, :location, :text,)
+    params.require(:document).permit(:creator_id, :content, :description, :project_id, :start_at, :end_at, :location, :text, :lock_version)
   end
 
   def parse_tag_names(tag_names)
@@ -133,4 +143,10 @@ def build_sort_query_with_default(param)
       "start_at DESC"
     end
   end
+
+  def get_form_data
+    @users = User.where(active: true)
+    @projects = Project.all
+    @tags = Tag.all
+  end
 end
diff --git a/app/controllers/tasks_controller.rb b/app/controllers/tasks_controller.rb
index a946031c..f2945654 100644
--- a/app/controllers/tasks_controller.rb
+++ b/app/controllers/tasks_controller.rb
@@ -48,7 +48,7 @@ def create
     @task = current_user.tasks.build(task_params)
     parse_tag_names(params[:tag_names]) if params[:tag_names]
 
-    if @task.save!
+    if @task.save
       matched = task_params[:description].match(/\[AI([0-9]+)\]/)
       if matched != nil
         ActionItem.find(matched[1]).update(task_url: tasks_path + "/" + @task.id.to_s)
@@ -59,6 +59,8 @@ def create
       end
     else
       respond_to do |format|
+        flash.now[:danger] = 'タスクの作成に失敗しました．'
+        get_form_data
         format.html { render :new, status: :unprocessable_entity }
         format.json { render json: @task.errors, status: :unprocessable_entity }
       end
@@ -67,17 +69,28 @@ def create
 
   # PATCH/PUT /tasks/1 or /tasks/1.json
   def update
-    parse_tag_names(params[:tag_names]) if params[:tag_names]
-    if @task.update(task_params)
-      respond_to do |format|
-        format.html { redirect_to @task, notice: "タスクを更新しました．" }
-        format.json { render :show, status: :ok, location: @task }
-      end
-    else
-      respond_to do |format|
-        format.html { render :edit, status: :unprocessable_entity }
-        format.json { render json: @task.errors, status: :unprocessable_entity }
-      end
+    @task.transaction do
+      parse_tag_names(params[:tag_names]) if params[:tag_names]
+      @task.update!(task_params)
+    end
+
+    respond_to do |format|
+      format.html { redirect_to @task, notice: "タスクを更新しました．" }
+      format.json { render :show, status: :ok, location: @task }
+    end
+  rescue ActiveRecord::StaleObjectError
+    flash.now[:danger] = 'このタスクは他のユーザーによって更新されました．'
+    get_form_data
+    respond_to do |format|
+      format.html { render :edit, status: :conflict }
+      format.json { render json: { error: flash[:error] }, status: :conflict }
+    end
+  rescue
+    flash.now[:danger] = 'タスクの更新に失敗しました．'
+    get_form_data
+    respond_to do |format|
+      format.html { render :edit, status: :unprocessable_entity }
+      format.json { render json: @document.errors, status: :unprocessable_entity }
     end
   end
 
@@ -106,7 +119,7 @@ def get_form_data
 
   # Only allow a list of trusted parameters through.
   def task_params
-    params.require(:task).permit(:assigner_id, :due_at, :content, :description, :project_id, :task_state_id)
+    params.require(:task).permit(:assigner_id, :due_at, :content, :description, :project_id, :task_state_id, :lock_version)
   end
 
   def parse_tag_names(tag_names)
diff --git a/app/views/documents/_form.html.erb b/app/views/documents/_form.html.erb
index a0a361cd..783efaf2 100644
--- a/app/views/documents/_form.html.erb
+++ b/app/views/documents/_form.html.erb
@@ -10,6 +10,8 @@
     </div>
   <% end %>
 
+  <%= form.hidden_field :lock_version %>
+
   <div class="mb-4">
     <%= form.label :content, "文書名", class: "form-label fw-bold mb-2" %>
     <%= form.text_field :content, class: "form-control", placeholder: "文書名を入力してください", required: true %>
diff --git a/app/views/tasks/_form.html.erb b/app/views/tasks/_form.html.erb
index 13e34352..cde1a94b 100644
--- a/app/views/tasks/_form.html.erb
+++ b/app/views/tasks/_form.html.erb
@@ -10,6 +10,8 @@
     </div>
   <% end %>
 
+  <%= form.hidden_field :lock_version %>
+
   <div class="mb-4">
     <%= form.label :assigner_id, "担当者", class: "form-label fw-bold" %>
     <%= form.select :assigner_id, @users.map { |u| [u.screen_name, u.id] }, {}, class: "form-select" %>
diff --git a/db/migrate/20260219050805_add_lock_version_to_documents.rb b/db/migrate/20260219050805_add_lock_version_to_documents.rb
new file mode 100644
index 00000000..a4e77d87
--- /dev/null
+++ b/db/migrate/20260219050805_add_lock_version_to_documents.rb
@@ -0,0 +1,5 @@
+class AddLockVersionToDocuments < ActiveRecord::Migration[7.2]
+  def change
+    add_column :documents, :lock_version, :integer
+  end
+end
diff --git a/db/migrate/20260225143559_add_lock_version_to_tasks.rb b/db/migrate/20260225143559_add_lock_version_to_tasks.rb
new file mode 100644
index 00000000..6166898b
--- /dev/null
+++ b/db/migrate/20260225143559_add_lock_version_to_tasks.rb
@@ -0,0 +1,5 @@
+class AddLockVersionToTasks < ActiveRecord::Migration[7.2]
+  def change
+    add_column :tasks, :lock_version, :integer
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index de436750..51d75092 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2023_08_24_085144) do
+ActiveRecord::Schema[7.2].define(version: 2026_02_25_143559) do
   create_table "action_items", force: :cascade do |t|
     t.integer "uid"
     t.string "task_url"
@@ -44,6 +44,7 @@
     t.text "location"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.integer "lock_version"
     t.index ["project_id"], name: "index_documents_on_project_id"
   end
 
@@ -91,6 +92,7 @@
     t.datetime "updated_at", null: false
     t.integer "tag_id"
     t.integer "task_state_id"
+    t.integer "lock_version"
     t.index ["project_id"], name: "index_tasks_on_project_id"
     t.index ["tag_id"], name: "index_tasks_on_tag_id"
     t.index ["task_state_id"], name: "index_tasks_on_task_state_id"