A light-source drive control unit divides each pixel of the image data into a plurality of subpixels, deletes certain subpixels from the image data in accordance with predetermined correction data, shifts remaining subpixels in the sub-scanning direction thereby obtaining reduced image data, and controls a plurality of light sources based on the reduced image data in such a manner that one line of the subpixels is formed with a light beam emitted from a corresponding one of the light sources.