In a full-color image forming apparatus including a light source for monochrome image formation that outputs light corresponding to monochrome image information and exposes a monochrome image forming photoreceptor and light sources for color image formation that output lights corresponding to color image information and expose photoreceptors for forming images of colors other than black, when the light source for monochrome image formation and the light sources for color image formation are caused to output the lights and a full-color image is formed, an optical output of the light source for monochrome image formation is set to become equal to or less than optical outputs of the light sources for color image formation, preferably, become less than the optical outputs of the light sources for color image formation.