To provide an image forming apparatus capable of obtaining an excellent image even when a crack is caused and realizing the long-time use of a seamless belt.

The existence of a positioning mark put on the outer circumferential surface side of the end in the width direction of an intermediate transfer belt and the existence of the crack on the intermediate transfer belt are detected, and a 1st optical sensor generates a detection signal. The crack is discriminated from the positioning mark according to the detection signal, and the number of the positioning marks discriminated within the time when the intermediate transfer belt makes a round is counted to grasp the position of the positioning mark. The positioning mark discriminated first after discriminating the caused crack is set as a reference mark, and the exposure start timing of an exposure means is controlled so that the prescribed position from the reference mark whose position is grasped and the edge part of a developer image may be aligned at a 1st transfer position.