An image forming apparatus restricts disturbance of a potential patch in the case where a potential sensor for detecting the potential patch is provided downstream of a developing device of a multiple developing roller type so as to enable stable reproduction of a high quality image for a long period. The apparatus includes a potential sensor provided downstream in the moving direction of the image carrier relative to the developing device for detecting a potential on the image carrier, and a controller for setting the developing bias to a value restricting disturbance of a potential portion as an object for potential detection by the potential sensor by the developer when the potential portion passes across the developing device.