To provide a light beam scanner which enables the scanning in a wide range by widenning the deflection half angle of a light beam to the extent exceeding 45°, and to dissolve the pitch unevenness caused by the plane tilt of the deflection reflective surface of the highly precise deflector.

This light beam scanner is equipped with a beam forming means having an optical system for converting the light beam emitted from a light source into parallel rays of light or a convergence light beam, a deflection means for deflecting the converted light beam to the main scanning direction on the surface to be scanned, a scanning image-formation optical system for imaging the light beam deflected by the deflection means on the surface to be scanned by scanning it at a deflection half angle of 45° or more, and a compensating optical system for the plane tilt of a cylindrical mirror with a concave surface having a positive refractive power only in the subscanning direction for compensating the pitch unevenness on the surface to be scanned caused by the plane tilt of the deflection means.