PURPOSE: To accurately adjusting an image density without having dispersion by changing the developing bias of an electrophotographic processing part by an image density varying means with an instruction from a host computer.

CONSTITUTION: A controller part 1 sends a command to set a value to the D/A 72 inside an engine driver part 2, thereto, according to the command sent from a host machine 56 to the controller part 1 setting the developing bias of the electrophotographic processing part. A CPU 59 inside the engine driver part 2 is set to data the D/A 72 based on the command, and changes the value of a reference voltage VrefB. Thus, the image density varying means 72 changes the reference value of the developing bias with the instruction from the host computer 56, so that the image density of the electrophotogrphic process part is accurately adjusted without having the dispersion.