An aspheric mirror has an aspheric reflective surface symmetrical with respect to an optical axis that goes through the reflective surface and a reference plane provided in the reflective surface. The optical axis may go through the reflective surface at a non-center position on the reflective surface. The reference plane may be perpendicular to the optical axis. The reference plane may be flat enough for a beam to be reflected from the reference plane in almost specular reflection when emitted to the reference plane.