Patent ID: 8260160

Claim:
A color image forming apparatus comprising: photosensitive members; chargers for charging surfaces of the respective photosensitive members; an exposure device for forming electrostatic latent images on the surfaces of the photosensitive members charged by the chargers; developing devices for developing the respective electrostatic latent images into toner images with developers comprising toner and carriers; toner concentration sensors for detecting toner concentrations of the developers in the respective developing devices; a voltage applying section for applying a developing bias voltage between each of the photosensitive members and each of the developing devices; a toner image bearing member for bearing toner images formed by the developing devices temporarily; a toner adherence amount sensor for detecting toner adherence amounts of the toner images held on the image bearing member; and a controller for calculating, with respect to the developing devices, individual optimal developing bias voltage ranges within which a target toner adherence amount can be achieved while the toner concentrations of the developers are within a predetermined toner concentration range from a lower limit to an upper limit, so as to specify a common optimal developing bias voltage range that are commonly suitable for all the developing devices to achieve the target toner adherence amount while the toner concentrations of the developers in the developing devices are within the predetermined toner concentration range, and for calculating individual target toner concentrations of the developers in the respective developing devices to achieve the target toner adherence amount while a voltage within the common optimal developing bias voltage range is applied between each of the photosensitive members and each of the developing devices.