A light scanning device deflects a plurality of light beams by a deflecting means performs divided scanning on a single scanning line on a photoconductor with the plurality of deflected light beams. When the scanning line is scanned, a detecting means detects a light beam passing through a predetermined position in front of a position where the light beam first strikes an image forming range on the photoconductor. An oscillating means oscillates a specified number of clocks in a time interval from when the light beam is detected by the detecting means until it is detected thereby again. An adjusting means makes adjustments so as to irradiate a plurality of light beams based on clocks oscillated by the oscillating means. In a time interval after the light beam is detected by the detecting means until it is detected thereby again, a plurality of light beams are irradiated based on the specified number of clocks. Therefore, even if unevenness occurs in the deflecting speed of the deflecting means, dots can be formed at the same intervals in scanning a plurality of scanning lines. Thus, it is possible to prevent discontinuous joints occurring in each of the images formed with a plurality of light beams.