To obtain a wide uniform beam by deciding a reflecting base surface having a reference focus to a reflector, determining a line passing through the whole points of give the same horizontal deviation to the emitted light, and changing the reflecting base surface so as to arrange a filament to horizontally diffuse the light running along this.

A reflector 20 of a head lamp for an automobile has two zones 21 and 22, and these zones are separated from each other by a curved transfer line LT. These zones have two surfaces, and the respective two surfaces generate a part of a light beam of whose boundary is determined by a cutoff line. The reflector 20 has an almost rectangular aperture. A light source has an axial directional filament 10 of a lamp so as to generate a chopped beam having the prescribed light distribution. A side zone 21 has at least a single groove S so as to horizontally diffuse the light. These respective grooves S are arranged between two lines having equal deviation of the zone 21. The whole points on this reflector give the same horizontal directional deviation on a level of a reflected light flux. A shape, a position and arrangement of an equal deviation line are determined by calculation.