An optical scanning device includes: a light source that emits light rays; an aperture member that adjusts a diameter of the light rays; a deflector including a plurality of reflecting surfaces that deflect the light rays; a scanning optical system that guides a light ray, of the light rays incident on the deflector and deflected by the deflector so as to be subjected to scanning onto a to-be-scanned surface; and a synchronization detector that performs synchronization detection by using a light ray, of the light rays, reflected to the aperture from the deflector is provided.