A resonance load circuit is supplied with the high-frequency voltage from the inverter circuit, and includes an inductor, a capacitor and a discharge lamp. The resonance load circuit is so that a declination with respect to the lighting frequency fs is set to a range from -20.degree. to 40.degree. in impedance when the discharge lamp is operating at rating. The CPU continuously generates a pulse voltage for driving on and off the switch elements at a cycle shorter than a lighting cycle of the discharge lamp based on program data and data stored in a memory, and makes pulse-width modulation with respect to an on-state width of the pulse voltage in accordance with a waveform change of a sinusoidal voltage corresponding to the lighting cycle, and further, supplies an approximately sinusoidal current to the discharge lamp using a high-frequency output from the inverter circuit.