To provide an image forming apparatus which is short-sized in terms of the whole device, dispensing with the reduction of the capacity of paper feeding/discharging trays and a printing performance.

In the image forming apparatus 1 constituted so that an electrifier 13, an exposure part 25, a developing unit 14 and a transfer unit 15 may be arranged in order in the rotating direction of a photoreceptor drum 12, an optical path 26 of laser light L reflected by a polygon mirror 21 of an optical unit 5 is returned, by arranging reflection mirrors 23 and 24 halfway from the polygon mirror 21 to the exposure part 25. Then, some of the optical unit 5 such as a laser oscillator and the polygon mirror 21, etc., are arranged under the exposure part 25. By reducing free space exists in the conventional apparatus, the device main body 8 of the image forming apparatus 1 is made shorter in height.