The apparatus is adapted to deflect a light beam from a laser light source for each of the color components by means of a deflection mirror surface which oscillates, thereby making the light beam reciprocally scan in a main scanning direction. In this apparatus, however, only a light beam SL which scans in a first direction (+X) of the main scanning direction is irradiated in an effective image region on a photosensitive member, so as to form a latent image thereon. The resultant latent image is developed to form a toner image. Since image formation is performed using only the light beam SL which scans in the first direction (+X), the images may be formed at the consistent density irrespective of the image types. Furthermore, the scanning directions of the light beams SL for all the color components are uniformly defined to be the first direction (+X), so that the toner images of the respective colors may maintain the consistent density.