PURPOSE: To prevent a misoperation in case of selecting a mode after a developing device is exchanged by inhibiting the selection of a mode for forming a color image with an action for setting the developing device housing black toner.

CONSTITUTION: A developing device discriminating circuit 20 detects that the toner is exchanged between the developing device 5Y and the developing device 5BK. When a user selects an image forming mode by operating a copying mode specifying circuit 25, the selected image forming mode is displayed by a display device 26. When the mode is going to be selected from unusable modes, an inhibiting mode setting circuit 27 is actuated so as not to actuate devices such as an image reader 2, a photosensitive drum 3, an image exposing device and a developing device, etc. Thus, in case of setting the developing device 5BK, the misoperation can be prevented and operability is improved by inhibiting a color mode.