Patent ID: 7092651

Claim:
An image forming apparatus that carries out image formation by primarily transferring an image electrophotographically formed on an image carrier onto a rotatably driven intermediate transfer member, and then secondarily transferring the images on said intermediate transfer member onto a recording medium, comprising: a controller that controls an image forming operation of primarily transferring the image onto said intermediate transfer member, according to a length of said intermediate transfer member in a circumferentially moving direction thereof and a variation of a predetermined parameter relating to said intermediate transfer member; a circumferential length detecting device that detects a circumferential length as the length of said intermediate transfer member in the circumferentially moving direction thereof; a signal generating device that generates an image formation start signal for a plurality of respective colors; a target value setting device that sets a target value of image formation timing to be input to said signal generating device based on the circumferential length detected by said circumferential length detecting device; and an offset value adding device that adds an offset value determined according to an expected load variation to the target value set by said target value setting device, wherein said circumferential length detecting device comprises a reference member detecting device that detects a reference member attached to said intermediate transfer member, and a measuring device that measures a time period elapsed from generation of a first detection signal acquired from said reference member detecting device to generation of a second detection signal acquired from said reference member detecting device as a result of circumferential movement of said intermediate transfer member.