A pair of additional reflectors are disposed face to face under a light source. The reflective surfaces of the respective additional reflectors are formed of paraboloids of revolution, with the first focal point of the reflective surface of a reflector as a focal point, and with an optical axis as the central axis of the revolution. As a result, retroreflective light from both reflectors is incident on the upper reflective region of the reflector. Thus, the spreading of the retroreflective light can be reduced based on the enhanced efficiency of utilizing the luminous flux of the light source, to the extent of the presence of the retroreflective light, as compared with the related art structure, wherein a spherical reflective surface centering on the light source is provided.