An optical scanner includes a light source (1) that emits a light beam, a light deflector (6), an incident optical system (5) that allows the light beam to impinge on the light deflector such that the width of the light beam in the main scanning direction is larger than the width of each deflecting surface of the light deflector in the main scanning direction, and a focusing optical system (8) that guides the light beam deflected by the light deflector to a surface to be scanned. The incident optical system and the focusing optical system are constructed such that aberrations occurring in the two optical systems when a light beam impinges on the surface to be scanned at an off-axis image height are asymmetrical on both sides of the center of the light beam and the directions of the aberrations are opposite to each other.