When a front fog lamp and a rear fog lamp are turned on by swinging an operating knob 4, a holder 18 climbs up to the crest portion 13b of a cam 13 and brings the tip of an engaging piece 22 into contact with the edge face of a coupling member 41. When an operating knob 5 is swung from an OFF position to an AUTO position in this state, the engaging piece 22 is urged by a coil spring 21 to resiliently stick into an arcuate engaging hole 45. When the operating knob 5 is swung back to the OFF position, an operating knob 4 is swung back by the engaging piece 22 in an interlocking mode. Then the holder 18 climbs down from the crest 13b of the cam 13 and causes the rear fog lamp to be turned off and only the front fog lamp is held ON.