In an optical scanning device, a light source includes a first light emitting point of an amount of light corresponding to a first mode and a second light emitting point of an amount of light corresponding to a second mode, and a control unit uses light beams emitted from a light emitting point of an amount of light corresponding to a mode set by a setting unit when the writing of image data is performed and uses light beams emitted from one prescribed light emitting point as light beams for synchronization detection, regardless of the mode set by the setting unit, when synchronization detection is performed before the writing of the image data is started.