PURPOSE: To provide an image forming device and the application control method of a developing bias voltage, capable of forming an excellent image without fogging and the transfer of carriers by controlling the timing of the application of a voltage to each developing unit in a rotary developing device with high accuracy.

CONSTITUTION: In the rotary developing device 4 provided with plural developing units 4a, 4b, 4c and 4d around a rotary body 4e and attaining development in such a manner that each developing unit is confronted with an image cgrrier 1 by the drive of a rotary shaft 6, a pulse inputted into a stepping motor 5 for rotating the rotary shaft 6 is counted by a pulse counter 14 and the timing of the application of the developing bias voltage from developing bias supply 15 to each developing unit is controlled based on the count value. Thus, the timing of the application of the developing bias voltage accurately coincides with the arrival of each developing unit at a fixed position in the rotational direction.