A light emitting reference point is set for a light source attached to a reflecting mirror having an opening on one end thereof, and it is assumed that a point light source is set on the reference point. Then, a reference surface formed as a set of half lines extending from the reference point to an arbitrary point on a front end edge of a socket arranged so as to interrupt rays of light from the point light source. Then, a partial surface of the basic surface of a reflective surface around the socket is arranged so that the partial surface projects more frontward than the reference surface, and a curved form of the partial surface is arranged so that the rays of light emitted from the point reach everywhere on the surface. Then, a group of paraboloids of revolution formed by a number of paraboloids of revolution whose focal distances are different from one another, is prepared, and intersecting lines between the basic surface and the group of paraboloids of revolution are obtained. Finally, a number of reflecting steps are formed on the basic surface between adjacent ones of the intersecting lines. The reflecting steps are defined by parts of the individual paraboloids of revolution.