When an magnified and synthesized image is finally printed on a printing medium based on an original image and a character font data, linear magnifying processing is executed with the aid of a software on the host computer side for the original image optically read by a scanner. In addition, vector magnifying processing is executed for font data likewise with the aid of a software, and moreover, synthesizing processing is executed for the font data. At this time, forming processing such as edge emphasizing or the like can be executed after completion of the vector magnifying processing. Synthesizing data are additionally magnified in a printer with the aid of a hardware, and subsequently, the magnified and synthesized image is outputted to the printer for performing a printing operation by the latter. An magnifying rate applicable to the host computer and the printer can be determined such that productivity of the image forming system is maximized.