diff --git a/app/controllers/dashboard/orders_controller.rb b/app/controllers/dashboard/orders_controller.rb
index 2c31bf5..8d7febc 100644
--- a/app/controllers/dashboard/orders_controller.rb
+++ b/app/controllers/dashboard/orders_controller.rb
@@ -16,6 +16,34 @@ def new
   end
 
   def create
-    # Write Code Here
+    service = OrderCreationService.new(
+      order_params: order_params,
+      token: params[:order][:token]
+    )
+
+    begin
+      @order = service.call
+      redirect_to dashboard_orders_path, notice: "注文が作成されました"
+    rescue OrderCreationService::ReservationAlreadyCompletedError => e
+      flash[:alert] = e.message
+      redirect_to dashboard_orders_path
+    rescue OrderCreationService::InsufficientStockError => e
+      @order = Order.new(order_params)
+      @order.name = Item.find(params[:order][:item_id]).name
+      flash.now[:alert] = e.message
+      @items = Item.all.order(:name)
+      @users = User.all.order(:name)
+      render :new
+    rescue ActiveRecord::RecordInvalid
+      @items = Item.all.order(:name)
+      @users = User.all.order(:name)
+      render :new
+    end
+  end
+
+  private
+
+  def order_params
+    params.require(:order).permit(:reservation_id, :user_id, :item_id, :email)
   end
 end
diff --git a/app/services/order_creation_service.rb b/app/services/order_creation_service.rb
new file mode 100644
index 0000000..422ffc6
--- /dev/null
+++ b/app/services/order_creation_service.rb
@@ -0,0 +1,56 @@
+class OrderCreationService
+  def initialize(order_params:, token:)
+    @order_params = order_params
+    @token = token
+  end
+
+  def call
+    reservation = Reservation.find(@order_params[:reservation_id])
+    item = Item.find(@order_params[:item_id])
+
+    validate_preconditions!(reservation, item)
+
+    order = build_order(item)
+
+    Order.transaction do
+      order.save!
+
+      # 決済APIを実行
+      payment_result = PaymentApiClient.execute(token: @token, amount: item.price)
+
+      # Paymentレコードを作成
+      order.create_payment!(
+        payment_id: payment_result[:payment_id],
+        amount: payment_result[:amount]
+      )
+
+      reservation.update!(status: :completed)
+
+      # 在庫数を1減らす
+      item.decrement!(:stock)
+    end
+
+    order
+  end
+
+  private
+
+  def validate_preconditions!(reservation, item)
+    if reservation.completed?
+      raise ReservationAlreadyCompletedError, "この予約はすでに処理済みです。"
+    end
+
+    if item.stock < 1
+      raise InsufficientStockError, "在庫が不足しています。"
+    end
+  end
+
+  def build_order(item)
+    order = Order.new(@order_params)
+    order.name = item.name
+    order
+  end
+
+  class ReservationAlreadyCompletedError < StandardError; end
+  class InsufficientStockError < StandardError; end
+end
diff --git a/app/views/dashboard/orders/new.html.erb b/app/views/dashboard/orders/new.html.erb
index 4908e4f..a3a6d8d 100644
--- a/app/views/dashboard/orders/new.html.erb
+++ b/app/views/dashboard/orders/new.html.erb
@@ -25,6 +25,7 @@
 <h2>注文情報</h2>
 <%= form_with model: [:dashboard, @order], data: { turbo: false } do |form| %>
   <%= form.hidden_field :reservation_id %>
+  <%= form.hidden_field :email %>
 
   <div class="field">
     <%= form.label :item_id, "購入商品" %>
diff --git a/test/controllers/dashboard/orders_controller_test.rb b/test/controllers/dashboard/orders_controller_test.rb
index 8965865..4acd8e9 100644
--- a/test/controllers/dashboard/orders_controller_test.rb
+++ b/test/controllers/dashboard/orders_controller_test.rb
@@ -26,6 +26,7 @@ class Dashboard::OrdersControllerTest < ActionDispatch::IntegrationTest
         reservation_id: reservation.id,
         item_id: item.id,
         user_id: user.id,
+        email: reservation.email,
         token: token
       }
     }
@@ -53,4 +54,99 @@ class Dashboard::OrdersControllerTest < ActionDispatch::IntegrationTest
     item.reload
     assert_equal initial_stock - 1, item.stock
   end
+
+  test "should not create order when reservation status is already completed" do
+    reservation = reservations(:one)
+    reservation.update!(status: "completed")
+    item = items(:one)
+    user = users(:one)
+    token = SecureRandom.alphanumeric(32)
+
+    assert_no_difference("Order.count") do
+      post dashboard_orders_path, params: {
+        order: {
+          reservation_id: reservation.id,
+          item_id: item.id,
+          user_id: user.id,
+          email: reservation.email,
+          name: reservation.name,
+          token: token
+        }
+      }
+    end
+
+    assert_redirected_to dashboard_orders_path
+    assert_equal "この予約はすでに処理済みです。", flash[:alert]
+  end
+
+  test "should update reservation status from pending to completed on successful order creation" do
+    reservation = reservations(:one)
+    reservation.update!(status: "pending")
+    item = items(:one)
+    user = users(:one)
+    token = SecureRandom.alphanumeric(32)
+
+    assert_equal "pending", reservation.status
+
+    post dashboard_orders_path, params: {
+      order: {
+        reservation_id: reservation.id,
+        item_id: item.id,
+        user_id: user.id,
+        email: reservation.email,
+        token: token
+      }
+    }
+
+    reservation.reload
+    assert_equal "completed", reservation.status
+  end
+
+  test "should decrease item stock by 1 on successful order creation" do
+    reservation = reservations(:one)
+    item = items(:one)
+    user = users(:one)
+    initial_stock = item.stock
+    token = SecureRandom.alphanumeric(32)
+
+    post dashboard_orders_path, params: {
+      order: {
+        reservation_id: reservation.id,
+        item_id: item.id,
+        user_id: user.id,
+        email: reservation.email,
+        token: token
+      }
+    }
+
+    item.reload
+    assert_equal initial_stock - 1, item.stock
+  end
+
+  test "should not create order when item stock is less than 1" do
+    reservation = reservations(:one)
+    item = items(:one)
+    item.update!(stock: 0)
+    user = users(:one)
+    token = SecureRandom.alphanumeric(32)
+
+    assert_no_difference("Order.count") do
+      post dashboard_orders_path, params: {
+        order: {
+          reservation_id: reservation.id,
+          item_id: item.id,
+          user_id: user.id,
+          email: reservation.email,
+          name: reservation.name,
+          token: token
+        }
+      }
+    end
+
+    assert_response :success
+    assert_equal "在庫が不足しています。", flash[:alert]
+
+    item.reload
+    assert_equal 0, item.stock
+  end
 end