PURPOSE: To project a light vertically to a scan face without curvature of field by arranging a parabolic mirror having a concave after a combinational lens consisting of the first W the fifth lenses so that the focus of this mirror coincides with the image of a reflective face of a rotary polyhedral mirror due to the combinational lens.

CONSTITUTION: A parallel luminous flux from a light source is reflected on a specular face 2a of a rotary polyhedral mirror 2 and is made incident on a combinational lens 3. The combinational lens 3 consists of the first W the fifth lenses 3a, 3b, 3c, 3D, and 3E arranged at intervals, and the parallel luminous flux 1 from the combinational lens 3 is made incident on a parabolic mirror 4 arranged at the rear of the combinational lens 3, and the light reflected from the parabolic mirror 4 is reflected on a plane mirror 5, which is arranged at an angle of inclination between the combinational lens 3 and the parabolic mirror 4, and is projected to a scan face 6 arranged under the mirror 5. In this case, the focus of the parabolic mirror 4 is made incident with the image of the specular face 2a of the rotary polyhedral mirror 2 to project the light.