An optical scanning device which performs an exposing operation with respect to a photosensitive material such as a printing sheet, etc., by scanning with laser beams in respective colors modulated according to image data is arranged so as to modulate light beams in respective colors projected from respective light sources of a red LD, a green-color SHG laser unit and a blue-color SHG laser unit by AOMs according to image data. Then, the clock frequency control section adjusts an exposure scanning clock for each of the AOM drivers for driving these AOMs individually. It is therefore possible to suppress an aberration in color caused by a shift in light beams in respective colors which are to be projected onto the same target position.