To prevent decrease in light emitting quality level of a main lamp and to simultaneously obtain a sufficient quantity of light from an auxiliary light by eliminating simultaneous lighting of the main lamp and the auxiliary light source, and to enable detection of a state where the main lamp cannot be turned on.

An auxiliary light source control circuit 5 turns on or off an auxiliary light source 3 according to a result of detecting a tube current flowing through a main lamp 2 conducted by a tube current detection circuit 4. When the auxiliary light source 3 is turned on, the main lamp 2 is irradiated with this auxiliary light source 3, and the main lamp 2 is also turned on. Then the tube current flowing through the main lamp 2 is detected by the tube current detection circuit 4, and the auxiliary light source control circuit 5 turns off the auxiliary light source 3.