PURPOSE: To improve the eddy flow distribution in a lamp chamber by providing two respiratory holes in the horizontal direction, one on a center point M of the rear surface of a lamp housing, and the other on the lower part of a middle point (m) of an end point N of the horizontal cross section including the point M, so as to constantly maintain the respiratory condition.

CONSTITUTION: A light source 4 is put on a center point M of a lamp chamber surrounded by a lamp housing 3a. The chamber is cut in the horizontal surface that includes the point M. The end of the side of the cross-sectional part of irregular shape (front side) 3e is defined as N, and a middle point (m) is determined between the two points M and N. Two respiratory holes 10a, 10b are provided in the horizontal direction on the lower part of the middle point (m). While at least one hole continues to be in a respiratory state, the positions of the two respiratory holes are correspondent to the region of weak eddy flow in the lamp housing, and are also close to the ascending draft along the inner wall surface of the lamp housing, the entire eddy flow distribution in the lamp chamber is improved by the two respiratory holes.