In a light chamber formed in a housing case and a lens cover placed at a front part of the housing case in a headlamp assembly, a projection lens, a shade and a light source are arranged in line from the front side of the light chamber along an optical axis of the light source. A radiating member has radiating fins of a plate shape vertically placed in the light chamber, and radiates heat energy generated by the light source to surrounding air. A slit composed of a vertical slit part and an inclined-slit part is formed in each of the radiating fins. The formation of the slit generates a change of heat capacity and a temperature difference at upper and bottom parts of each of the radiating fins. The incline-slit part is extended from the top of the vertical slit part to the upper part of the radiating fin.