PURPOSE: To vary power with simple lens constitution by varying the image forming power by the displacement of a light source and an image forming lens and collimating luminous flux from the image forming lens into parallel luminous flux by a cylindrical lens in a direction corresponding to a main scanning direction.

CONSTITUTION: The image forming lens 12 and cylindrical lens 14 are arranged between a light source 10, e.g. LD array and a polarizing means 16. The image forming lens 12 is provided on the side of the light source 1 and the cylindrical lens 14 has negative power. The light source 1 and image forming lens 12 are displaced in the direction of the optical axis. Further, the cylindrical lens 14 is provided at a fixed position so that its generating line direction, i.e. direction where there is no power corresponds to a subscanning direction. The magnification is varied by varying the image forming power of the image forming lens 1 by the displacement of the light source 1 and image forming lens 12. In this case, the light source 1 and image forming lens 12 are displaced so that the light from the image forming lens 12 is collimated into parallel luminous flux by the cylindrical lens 14 in the direction corresponding to the main scanning direction. Thus, the power is varied with the simple lens constitution.