The colors used for forming an image are limited in advance to a predetermined number of color patterns called representative colors; on the host side the image data received is divided into blocks, for each of which a representative color is determined in a representative color code conversion unit; the representative colors and their associated representative color codes are transmitted to the printer side; and on the printer side the representative color codes are converted into dot patterns in a pattern conversion unit for each pass before being output. The above procedure allows a series of processing--resolution conversion, masking/UCR processing, output .gamma.-correction and binarization processing--to be realized with a simple configuration.