This invention has as its object to provide an image forming apparatus which can provide a color image free from any color misregistration with low cost, and a light scanning unit suitable for the apparatus. The apparatus includes a finite lens and a cylinder lens for giving predetermined characteristics to light beams emitted by a plurality of light sources, a laser synthesis mirror unit for synthesizing laser beams passing through the lenses, a deflection unit for deflecting the synthesized light beams, first and second image-forming lenses for adjusting the aberration characteristics, at the imaging position, of the deflected light beam, one or three mirrors for outputting the light beam passing through the image-forming lenses at predetermined positions, and can provide a color image free from any color misregistration.