Patent ID: 7835670

Claim:
An image forming apparatus, comprising: a plurality of image forming units provided in parallel, and configured to form a color toner image at each image forming unit, the plurality of image forming units including a black image forming unit configured to form a black toner image, and a plurality of color image forming units configured to form each color toner image; an intermediate transfer device configured to primarily transfer a color toner image formed by superimposing the toner images formed at each image forming unit; and a secondary transfer unit configured to transfer the color toner image to a transfer paper, wherein the color image forming units include a non-magnetic-contact-type single-component development device to develop each color toner image with a single-component developer, and the black image forming unit is provided closest to the secondary transfer unit and includes a contact-type two-component development device to develop a black toner image with a two-component developer, each color image forming unit determines a threshold value for a size of an output image area, the color image forming unit performs a development process with a predetermined toner amount after image forming, and a bias voltage having a reverse polarity to a voltage for transferring to the intermediate transfer device is applied to the developed toner while the developed toner passes through a primary transfer nip when the size of the output image area size is less than the threshold value determined by each of the color image forming units.