A resonant transformer is disposed at a stage following an inverter for inverting the polarity of a voltage applied to a discharge lamp. A parallel resonant circuit is formed by a resonant capacitor and a primary winding of the resonant transformer, and a periodic voltage applying section is connected to the parallel resonant circuit. The periodic voltage applying section is operated at a frequency that causes resonance in the parallel resonant circuit, thereby applying a high AC voltage to the discharge lamp in a period in which discharge is started in the discharge lamp.