To make a color machine configuration and a monochromatic machine configuration only correspond with each other by changing method of use of buffer memories.

After inputted image information is digitally converted and those image signals are processed by an image data control part 13 into image signals usable for image formation, they are transmitted through an image memory access control part and stored in a memory module; when a printing operation is performed, they are read out and sent out to an image processing unit side. Further, this image processor is able to change the use mode of a 1st buffer memory 206a for transfer from the image data control part 13 to the memory module 25 and the use mode of a 2nd buffer memory 206b for transfer from the memory module to the image data control part 13 between the color machine configuration and the monochromatic machine configuration.