This invention relates to an MFP having copy and scanner functions. The MFP in which image data flows from a line sensor to a DRAM via an A/D conversion unit and a reading control unit (RCU) and then from an image processing unit (IPU) to a printer via the DRAM has the following arrangement. Both the RCU and the IPU can execute a shading process. In the CM, the RCU executes the shading process. Then, the image is compressed and stored in the DRAM. In the SM, the RCU does not execute the shading process, and the image is temporarily stored in the DRAM without compression. The IPU reads out image data of each rectangle from the DRAM together with shading data, executes the shading process and various kinds of image processing, and outputs the image data to the DRAM.