The memory area of an image memory 3 is divided into a plurality of areas (PL0 - PL3) for separetely storing yellow, magenta, cyan and block data constituting a full color image, and the stored color data are read out to form a full color image 3a on a sheet of paper. In forming a mono color image, mono color data are stored in the entire memory area. An image forming apparatus is thus realized which is capable of arbitrarily composing images with a small memory capacity.