An optical scanner includes an inductor, an electric supply unit, an emitter, a mirror, a first receiving element, a second receiving element, and a measuring unit. The electric supply unit applies a supply voltage to the inductor alternately in opposite applying directions. The mirror reflects a laser beam to scan a photosensitive body within a scanning range. Each of the receiving elements is positioned at one end of the scanning range. The measuring unit measures the scanning time taken for the laser beam to be detected by the second receiving element after the beam is detected by the first receiving element. When the mirror scans the photosensitive body along a first primary scanning line after the mirror scans the body along a second primary scanning line, the electric supply unit varies the supply voltage according to the scanning time measured for the second primary scanning line.