An inside unit (54) is housed in the unit receiving section 81 of a lamp socket (1). The central terminal (3) is connected to the high-voltage output terminal (76) of the inside unit (54) and mounted in the terminal aperture (11A) of a terminal mount (11) at the center of the lamp socket (1). The low-voltage output terminal (77) of the inside unit (54) is connected to the side terminal (4) of the lamp socket (1).