PURPOSE: To obtain both a monochromatic and a two-color mixed copy of the same picture quality with a three-color mixed copy by controlling the heat time of fixation according to the number of mixed colors.

CONSTITUTION: A controller 16 is controlled through a CPU14d, etc., according to a color mode specified on a keyboard 13, and the rotating speed of a motor which drives the heat roller of a fixing device 10 is varied to vary the driving speed of the heat roller, thereby controlling the heat time of a copy to be fixed so that the heat capacity of a copy is less and less as the number of mixed colors is smaller and smaller. Consequently, a monochromatic and a two-color mixed copy are reduced to the same in picture quality with a three-color mixed copy.