An image forming apparatus includes: a moving speed ratio change section which changes a circumferential speed ratio between a surface of a developer bearing member and a surface of an image carrier; and a memory section which stores a target value of fog toner. After the circumferential speed ratio has been changed by the moving speed ratio change section, a controller determines a fog control parameter based on comparing the value detected by the fog toner detection section with the target value stored in the memory section. After then, and the controller controls the moving speed ratio change section to return the circumferential speed ratio to the circumferential speed ratio prior to the change.