There is provided a technique in which a retracting operation of a bundle of sheets from a stapler after a staple process can be stably performed irrespective of the number of sheets constituting the bundle of sheets or the transport resistance. When the bundle of sheets subjected to the staple process is moved by a stacker in a direction of retracting from the stapler, an alignment roller is caused to come in contact with the bundle of sheets held by the stacker and to assist the movement of the bundle of sheet.