PURPOSE: To select desired image density and to obtain the excellent image density by controlling the rotational speed of a rotating body corresponding to an image forming mode inputted from an input means.
CONSTITUTION: In a controller 90, the respective keys of a control panel 70 are electrically connected to a main processor (CPU) 901. Then, the CPU 901 controls a sub-CPU 903 and makes it output a standard frequency. The standard frequency from the sub-CPU 903 is inputted to a comparison control circuit 905. By the control circuit 905, a control voltage is outputted so as to make a motor rotation detection pulse from a rotation sensor 95 coincide with the standard frequency. By a motor driving circuit 907, the rotation of a main motor 93 is controlled according to the control voltage. Then, the rotating body of a developing means is set to an optimum revolving speed according to the image forming mode and developing according to the image forming mode is executed. Thus, the desired image density according to the image forming mode is selected and the excellent image is obtained regardless of the type of an original.