To provide a scanner with which laser beams emitted from a plurality of laser light emitting parts are made incident on an optical deflection means with high accuracy in a sub-scanning direction and the accurate deflection and scanning of a plurality of laser light beams are assured, and to provide an image forming apparatus equipped with the scanner.

In a laser irradiation optical part 18, a second slit plate 28 on which two second slits 31 arranged in parallel with a distance in a sub-scanning direction Y are opened is provided between a reflection mirror 17 which synthesizes the optical paths of two laser light beams emitted from laser light emitting parts 24 which are perpendicularly arranged as a pair so as to coincide in a main scanning direction X, and a cylindrical lens 29 for converging two laser light beams with which a polygon mirror 17 is irradiated in the sub-scanning direction Y. The error of the two laser light beams made incident on the polygon mirror 17 is reduced not only in the main scanning direction X but also in the sub-scanning direction Y by respective second slits 31.