To minimize possibility of deforming or damage in a body bottom face portion to which a lighting circuit case provided with the discharge bulb is to be attached.

A metal plate 18 to which four weld bolts 20 are welded is provided on an inner face of a body bottom face portion 12a so as to penetrate each of four through holes 12b and 32b through which each of the weld bolts 20 is formed at a body bottom face portion 12a and a lighting circuit case 32, a nut 36 is securely fixed to each weld bolt 20, and whereby the lighting circuit case 32 is mounted on the body bottom face portion 12a. Thus, the body bottom face portion 12a is pinched and held by the lighting circuit case 32 and the metal plate 18, and stress concentration to the body bottom face portion 12a is prevented.