To inexpensively and efficiently prevent the deterioration in image quality without deteriorating the efficiency of utilizing light by making it possible to easily and adequately adjust bending and tilting of scanning lines formed by scanning with a light beam, such as a laser beam.

A knife edge-like projection 15a is formed in the central part at the base of a long-sized plastic lens 15. The projection 15a is supported by a projecting bar 19a having a top face of a V-groove shape projected at a housing 19. The top ends of supporting angle portions19b and 19c of the housing 19 are provided with adjusting screws S1 and S2 screwed and inserted thereto in correspondence to two points at both ends in the upper part of the lens 15, by which these portions are pressed and supported to two points at both ends in the longitudinal direction of the lens 15 in contact therewith. The lens 15 is bent and adjusted to a projecting shape upward around the projection 15a as a fulcrum by tightening the adjusting screws S1 and S2.