When an error occurs while a job flow including a plurality of steps is executed, an electronic signature is added to image data processed at a step that has already been performed, and the image data to which the electronic signature is added is stored. At that time, an electronic certificate expiring at the same date as the expiration date of the job flow is generated and added to the electronic signature. Before restarting execution of the job flow where the error occurred, the electronic signature added to the image data is verified and a user is notified of the verification result. A determination is then made whether execution of the job flow where the error occurred should be restarted according to an operation performed by the user in response to the notification.