The present invention provides an image processing apparatus that realizes fast image processing in a distribution system equipped with a single mass storage device that reduces the cost for required hardware, and a method for controlling such an image processing apparatus. To accomplish this, the image processing apparatus includes a mainboard, a subboard connected to the mainboard via a bus, and a hard disk 6 that is connected to the subboard and indirectly connected to the mainboard via the subboard and that stores image data. When an access request to the hard disk is generated, in the case of data transfer between the hard disk and the mainboard, normal access is employed. On the other hand, fast access is employed in the case of data transfer between the hard disk and the subboard.