A reflector 10 is formed so as to have a long side along a scanning direction of a ray bundle. The reflector 10 includes a thin-plate-shaped reflector body having one plane to be a reflection plane 20 and upper and lower ribs 21a and 21b each extending from the reflector body in a back side. The reflector plane 20 is formed of a curved plane having positive power at least in the scanning direction, i.e., a so called free-form plane. The reflector 10 is supported by protrusions 31, 32 and 33 at three points, i.e., first, second and third support points S1, S2 and S3 located in the vicinities of one end portion of the reflector, the other end of thereof, and a center portion thereof, respectively. A distance L1 between an imaginary straight line V1 joining the first and second support points S1 and S2 and the third support point S3 is larger than a sag L2 of the reflector 20. The center of gravity G of the reflector is located inside of an imaginary triangle V3 joining the first, second and third support points S1, S2 and S3.