An image forming apparatus is provided that reduces main scanning jitter with a simple configuration and performs light amount control with high accuracy. The image forming apparatus includes: a laser emitting luminous flux; a main-scanning aperture portion shaping the luminous flux; a beam splitter splitting the luminous flux passed through the main-scanning aperture portion into a reflected beam and a transmitted beam; a rotary polygon mirror deflecting the transmitted beam so that the transmitted beam scans the surface of a photosensitive drum; and an optical box in which the laser, the main-scanning aperture portion, the beam splitter and the rotary polygon mirror are disposed. The main-scanning aperture portion is disposed so as not to block a deflected and transmitted beam. The beam splitter abuts against the main-scanning aperture portion so as not to block a deflected and transmitted beam deflected, and is positioned by abutting against the main-scanning aperture portion.