To lengthen actual life and to reduce the frequency of replacement by installing two sets of independent filaments and two sets of circuit terminals connected to the filaments, and lighting another filament when the current use filament is not lit.

Lead-in wires 11a, 11b, filaments 12a, 12b, and first and second filament parts A, B are installed, and circuit terminals A1, B1 for the filament parts A, B constituting one set with two pins are installed in a synthetic resin insulator 19. A bulb 18 is inserted into a two-circuit outlet 3 having circuit terminals A2, B2 connected to a lighting automatic switching unit 4 with connecting wires 3a, 3b. The lighting automatic switching unit 4 supplies power to another filament 12b to light it, for example, when the filament 12a is not lit. The lighting automatic switching unit 4 has a means for switching power supply from a no-lighting filament to another filament when no-lighting of the filament part is detected.