A capacitor 42 (C1) of a first bootstrap circuit 4 for maintaining the ON state of a first switching device 61, one of the two switching devices disposed on a higher potential side of first DC voltage V1, is not only charged with second DC voltage V2, but also supplied with a charging current from third DC voltage V3 on a secondary winding n2 side of a transformer 22, and maintains the ON state of the first switching device 61 for a long time with the charge of both of them. This makes it possible to fix the polarity of the voltage to be applied to the discharge lamp 8 to the single side polarity closer to the DC output operation.