An image forming apparatus includes a plurality of photosensitive media, a light source unit which comprises a plurality of light sources, a polygon mirror which deflects a plurality of beams output from the plurality of light sources into the plurality of photosensitive media using a plurality of reflective surfaces, a beam detector which receives beams reflected from the polygon mirror during a rotating process of the polygon mirror, and outputs a beam detection signal, and a horizontal sync signal generator which receives the beam detection signal and counts beam reflecting times during which the beams are reflected from the plurality of reflective surfaces, and compares the plurality of counted beam reflecting times with the compensation values calculated for the reflective surfaces, respectively, generates a horizontal sync signal for a corresponding reflective surface, and provides the horizontal sync signal to the light source unit.