Any one of a plurality of modes such as a scanner function, a copy function, a facsimile function and the like can be set, and an operation mode of an image reading unit at the time of its image reading operation is judged. A return speed of the image reading unit for returning to a reference position after completion of the scanning is set at a speed corresponding to the operation mode. As a result, a machine sound generated when the image reading unit moves does not cause a user to feel uncomfortable, and the waiting time until the next document reading can be shortened.