Patent ID: 7555232

Claim:
An image forming apparatus comprising an image carrying member, a plurality of developing devices each capable of developing an electrostatic latent image formed on said image carrying member, a development bias applying device provided for each of said plurality of developing devices, a charge bias applying device provided for each of said plurality of developing devices and a control unit controlling said charge bias applying device provided for each of said plurality of developing devices, wherein each of said plurality of developing devices includes a developer carrying member carrying and transferring developer to a developing region developing said electrostatic latent image formed on said image carrying member, and a charging member being in contact with the developer carried on said developer carrying member and charging the developer; said development bias applying device applies an AC development bias voltage including an AC voltage to said developer carrying member; said charge bias applying device applies to said charging member an AC charge bias voltage having a phase synchronized with said AC development bias voltage and representing a charge bias voltage value causing an offset potential difference with respect to said AC development bias voltage in a phase on either one of positive and negative sides; said charge bias voltage applied by the charge bias applying device provided for each of said plurality of developing devices and causing said offset potential difference has the same voltage value as the charge bias voltages applied by the charge bias applying devices of the other charge bias applying devices; and said control unit controls said charge bias applying device provided for each of said plurality of developing devices according to at least one of factors affecting a charging property of the developer transferred to said developing region, and thereby controls said same charge bias voltage value to provide said offset potential difference with respect to said AC development bias voltage in a phase on either one of positive and negative sides causing the developer to bear a regular charge amount for developing the electrostatic latent image.