In an optical scanning device, a light-source driving unit drives a light source to modulate a light beam in units of single dot, a deflecting unit deflects the light beam, and a scanning optical system guides the deflected light beam to a scanning surface having an active writing area for forming an image. A static beam spot size, which is a beam spot size of the light beam on the scanning surface when the light beam passes a predetermined image height, is set such that a scanning beam spot size, which is a beam spot size of the light beam on the scanning surface when the light beam is moved to scan during the light beam exposes a single dot, is kept substantially constant.