A multibeam scanner including a light source for emitting a plurality of light beams, a deflector for deflecting the plurality of light beams emitted from the light source, and a scanning optical system for guiding the plurality of light beams deflected by the deflector to a scan surface. A first light shielding member for determining one end of the diameter of at least one of the plurality of light beams and a second light-shielding member for determining the other end of the diameter of the at least one of the plurality of light beams are disposed apart from each other in a direction in which the light beams propagate and between the light source and the deflector. The first light-shielding member and the second light-shielding member are used to limit the diameter of the at least one of the plurality of light beams.