There is an image forming apparatus which forms a visible image (toner image) on an image carrier. The image forming apparatus is provided with a photoconductor and a developing device. The image forming apparatus forms an electrostatic latent image on the photoconductor based on image information. The developing device develops the electrostatic latent image with toner. A two-component developing type developing device is provided with developing agent obtained by mixing toner and carrier, a developing roller and a developing agent mixer. The developing roller contacts the developing agent with the photoconductor. The developing agent mixer stirs the developing agent during the developing process. In one type of two-component developing type, the developing agent mixer further moves the developing agent to a discharge port. The developing agent mixer discharges the developing agent little by little to the outside of the developing device during the developing process.
The image forming apparatus of one type of two-component developing type keeps the discharge amount of the developing agent constant when changing the process speed. Such an image forming apparatus is provided with a mechanism for keeping the discharge amount of the developing agent constant. As a result, the constitution of the apparatus becomes complicated. For example, the image forming apparatus reduces the developing roller linear velocity during the developing process in a thick paper mode. However, if the rotation speed of the developing agent mixer is reduced, the discharge amount of the developing agent cannot be kept constant. Thus, it is necessary to arrange a dedicated driving motor for the developing agent mixer in the image forming apparatus of one type of two-component developing type.