To detect an upper position of a belt in a circulating direction by a transmission type optical sensor without necessitating complicated wiring and a connector or the like connecting with a device main body even though a unit such as an intermediate transfer belt is freely attachably/detachably installed at the device main body.

A square hole 100 is provided in a reinforcing member 110 on a right side in the advancing direction of the intermediate transfer belt 5a, and a rocking flag 101 is provided at a belt frame so as to be capable of rocking by fitting in the rotary shafts of both ends, and the stopper part 101e of a sensor side flag 101b is pressed by a torsion coil spring 102 engaged with the belt side flag 101a of the rocking flag so as to butt against the wall of the belt frame. The flag 101b is slid to a circulating intermediate transfer belt, and is rocked by dropping in the square hole at the position of the square hole, and the square hole is detected because the flag 101a cuts the optical path of the transmission type optical sensor 103, so that the upper position of the belt in the circulating direction is detected.