A coupling lens collimates an optical beam emitted from a laser light source. An aperture shields a peripheral light flux area of the collimated optical beam. A phase adjusting element partially changes a phase of wavefront of the optical beam. An auxiliary aperture is formed on an outer side of a normalized aperture size in at least one of a main scanning direction and a sub-scanning direction. The phase adjusting element is formed in a parallel plate, and changes at least a phase of wavefront of a peripheral portion of the optical beam to compensate decreases of the beam spot size and the depth allowance caused by the auxiliary aperture.