PURPOSE: To prevent toner from depositing to the outside of an image forming area by impressing a voltage which inclinedly changes nearly corresponding to a rise part and a fall part from a developing bias power source provided with a time constant circuit.

CONSTITUTION: The time constant circuit constituted of a capacitor C, resistances R1 and R2, and a zener diode Vz, etc., is incorporated in the developing bias power source so as to change the developing bias voltage having inclination corresponding to the potential inclination of the electrostatic charging potential rise area and fall area. Thus, the inclined area where the voltage is gradually rises or falls at the time of starting or finishing impressing the developing bias voltage is formed, and the surface potential of a photosensitive body 1 does not cause the deposition of the toner in the rise area and the fall area. Therefore, the toner is prevented from depositing to an image non-forming area.