Abstract: An optical beam scanner is provided in which light emitted from a light source and transmitted through a slit formed in a diaphragm plate is made incident upon a polygonal mirror through a cylindrical lens having positive power in the sub-scanning direction. The light reflected by the polygonal mirror is converged onto an object surface to be scanned through an optical image forming system. A plane mirror is provided between the diaphragm plate and the polygonal mirror to reflect the light emitted from the light source toward the polygonal mirror. The optical elements provided between the light source and the optical image forming system are housed in a casing having a bottom surface perpendicular to the axis of rotation of the polygonal mirror. The light emitted from the light source toward the plane mirror is parallel with the bottom surface. The light incident upon the polygonal mirror and the light reflected by the polygonal mirror are separated in the sub-scanning direction.