An optical scanner includes a light source unit configured to emit light; an optical system configured to cause the light emitted from the light source unit to scan an object so as to form an image on the object; and a control unit configured to control the light source unit. The light source unit includes a plurality of light emitting units arranged in a sub scanning direction, and the control unit controls the light source unit to emit light at different positions on the object by using either n (n: positive integer) light emitting units or n+1 light emitting units selected from among the plurality of light emitting units, according to a positional shift amount of the image in the sub scanning direction.