PURPOSE: To reduce the reactive current contained in a resonance current and downsize an inductor, in an object equipped with an inverter circuit of LC series resonance type using a ballast capacitor.

CONSTITUTION: For a discharge lamp lighting unit, a half bridge circuit 4 is made of a pair of switching elements 2 and 3 connected to a DC power supply 1, and load circuits 8a and 8b, which have capacitors Ca and Cg as impedance through an LC series resonance circuit 5, are connected to the middle junction A between these switching elements 2 and 3 in a pair. And, an inductor L is connected in parallel to the former stage of the load circuits 8a and 8b so as to make it have inductive property, and the reactive currents by phase difference are reduced by offsetting the capacitive property on the side of load circuits 8a and 8b.