An image forming apparatus includes photosensitive drums including one for black, light source devices, a rotary polygonal mirror having deflecting surfaces with chamfers, input optical systems, imaging optical systems and a synchronism detecting system for detecting a synchronizing light beam for determining the timing for scanning each of the photosensitive drums in a main scan direction, wherein the synchronism detecting system detects a light beam for forming an image of a color different from the black, and wherein the timing as the synchronism detecting device obtains a synchronizing signal and the timing as a light beam from the input optical system, which is at the side where the synchronism detecting device is provided, is incident on the chamfer of the deflecting surface of the rotary polygonal mirror, are at least partly overlapping.