A multifunction peripheral capable of preventing discontinuation of a simultaneous double-sided reading process, caused by congestion of an image bus. An image transfer clock is controlled for transferring image data of a document sheet from a reading unit for reading both sides of the document sheet, to an image processing unit for performing image processing on data acquired via an image bus. When transfer of image data of one side of the document sheet is singly executed, a frequency of the image transfer clock is set to a predetermined frequency, and when transfer of image data of the other side of the document sheet, which is executed in succession to the transfer of image data of the one side, is to be executed in parallel with the image processing, the frequency is switched from the predetermined frequency to a lower frequency than the same.