Abstract: The light beam scanning device has a casing injection molded from magnesium or magnesium alloy as the material. A polygon mirror and a motor are sealed in the inside of the casing and a cover covering the latter. Since the thickness of the magnesium or magnesium alloy material can be reduced, heat generated from a motor is dissipated satisfactorily. Heat dissipating fins are formed to the casing to promote heat dissipation and enhance the strength of the casing.

Abstract: The light beam scanning device has a casing injection molded from magnesium or magnesium alloy as the material. A polygon mirror and a motor are sealed in the inside of the casing and a cover covering the latter. Since the thickness of the magnesium or magnesium alloy material can be reduced, heat generated from a motor is dissipated satisfactorily. Heat dissipating fins are formed to the casing to promote heat dissipation and enhance the strength of the casing.

Abstract: A light beam scanning optical apparatus which writes two lines at one scan on a photosensitive member with light beams emitted from laser diodes. The light beams are combined at a beam splitter. The beam splitter is supported on a surface including a semitransparent layer by total three balls provided on a base block.

Abstract: A laser beam scanning optical apparatus which has a laser source unit, a collimator lens, a cylindrical lens, a polygon mirror and a scanning lens with an f.theta. function. The collimator lens is a bifocal lens, and because of the bifocal collimator lens, a part of the laser beam is focused on a point before the surface of a photosensitive member, and another part of the laser beam is focused on a point after the surface of the photosensitive member. Also, the cylindrical lens can be a bifocal lens. In this case, even when the magnification ratio in the sub scanning direction is designed small, the focal length of the cylindrical lens does not have to be lengthened.