PURPOSE: To correctly replenish toner even in a simultaneous, two-color printing by controlling the toner density of only a developing unit forming a toner image according to the density of a reference toner image.

CONSTITUTION: Out of plural areas of an original, the largest one is detected, and the developing unit 4 or 5 developing the area forms a reference toner image on the surface of an electrostatic latent image carrier 2. The toner density of only the developing unit 4 or 5 forming a toner image is controlled according to the density of the reference toner image. Even if the simultaneous, two- color copying is carried out, a decrease in the toner density of a developing unit forming a small area is minimal, which hardly affects image quality. Thus, toner can be replenished correctly even in the multicolored copying machine performing the simultaneous, two-color printing.