A light scanner includes a semiconductor laser, a polygon mirror, a first imaging optical system for guiding a light beam from the laser to the deflection surface of the polygon mirror, a second imaging optical system of a single curved mirror for guiding the light beam from the polygon mirror to a photosensitive drum, and a photodiode for detecting the light beam scanned by the polygon mirror. The first imaging optical system, the polygon mirror, and the second imaging optical system are located at different positions in the sub-scanning direction so that the light beam from the first imaging optical system enters the deflection surface obliquely, and the light beam from the polygon mirror enters the curved mirror obliquely.