PURPOSE: To prevent a transfer belt from being soiled by an adhering toner from an image carrier and to maintain the functions of the transfer belt over a long period of time by selecting either of plural transfer rollers during transfer to correspond to transfer size.

CONSTITUTION: A transfer unit 8 has a transfer roller unit provided with three transfer rollers 83, 84, 85 having roller portions with different axial lengths, and selects either of the transfer rollers whose roller portion corresponds in length to the size of the transfer paper used. Therefore, the greater part of a transfer belt 86, where no transfer paper exists, is not pressed against an image carrier 4, and so soiling of the belt with an adhering toner from the image carrier 4 can be minimized. While transfer is not being carried out, the intervals among the transfer rollers of the transfer roller unit 82 are positioned to be opposite to the image carrier 4, thus forming a gap between the image carrier 4 and the transfer belt 86. Thus nonconformities, such as permanent set of the transfer belt 86 caused by constantly pressing the belt into contact with the image carrier 4, can be prevented.