A multifunction apparatus that is capable of executing a scan job and another image process in parallel without lowering a reading speed of a scanner unit. A reading unit reads a document according to an execution instruction for a predetermined image process. An image processing unit applies the predetermined image process to image data obtained by the reading unit. A clock controller controls an image transfer clock for transferring the image data to the image processing unit from the reading unit. The clock controller sets a frequency of the image transfer clock to a predetermined frequency in a case where the predetermined image process is executed independently. And the clock controller sets the frequency of the image transfer clock to a frequency lower than the predetermined frequency in a case where the predetermined image process is executed in parallel with another image process that differs from the predetermined image process.