A fourth lead wire (LW40), since arranged in a shading coverage, is prevented from exposure to the irradiation lights from a low-beam filament (51) by the effect of the shading function of a high-beam filament (52). The front end of a third support wire (SW3) in a semi-shading coverage, since bent to a curved surface, receives the irradiation lights from the low-beam filament (51) but with a minute area; and the light diffusion function of the bending curved surface acts thereupon. The lower end on the front end portion (SW20) of a second support wire (SW2), since positioned above the lower end of the low-beam filament (51), is prevented from appearing above a light-shade boundary line. In either use as a right-traffic light source bulb or as a left-traffic light source bulb, lights from the low-beam filament (51) and the high-beam filament (52) reach the reflecting surface (40) of a reflector (4) without passing through a rear-end sealed part of a glass envelop, eliminating optical-path changes in the rear-end sealed part, production of glare light, and light-distributional problems. This results in freedom from virtual image glare, so that a single light source bulb (5) can be used both as a left-traffic light source bulb and a right-traffic light source bulb.