An optical scanning device includes a reflector that is pivotable, a light-emitter that emits a detection light toward the reflector, a detector that receives the detection light reflected at the reflector, and a light-blocking unit that includes a slit wherethrough the detection light passes. The light-blocking unit is disposed on an optical path whereby the detection light reflected from the reflector advances to the detector. The slit has an aspect ratio where a length of the slit in a direction in which the optical path extends relative to a length of the slit in a width direction is equal to or greater than a predetermined value. The width direction is along a pivoting direction of the reflector, of an opening end portion of the slit. The detector is configured to detect a pivot angle of the reflector by receiving the detection light that passes through the slit.