Abstract: Stable developing can be made with no inferior image, irrespective of variations in factors relating to the developing function, which affect the developing function, such as a variation in surrounding circumstance, surface potentials of an electrostatic latent image carrier medium and a developer carrier medium or the like. When surface potential sensors serving as a means for detecting a factor relating to the developing function detect variations in the surface potentials of latent image carrier medium or developer carrier medium, voltages applied from power sources, to conductivity regulating member, developer carrier medium and electrically conductive developer supply member are controlled so that the volume of the developer sticking to the electrostatic latent image carrier medium is maintained to be constant.