A DC/AC converter 3 performs AC conversion and a boosting function upon the reception of a DC voltage. A control unit 6 controls the DC/AC converter 3 to perform the lighting control of a discharge lamp. The DC/AC converter includes an AC conversion transformer 7, switching devices 5H, 5L and a resonance capacitor 8, and drives the switching devices to produce series resonance in the capacitor 8 and an inductance component for the transformer 7 or the inductance device 9. Before the discharge lamp 9 is turned on, the drive frequency for the switching devices gradually nears a resonance frequency f1 to increase an unloaded output, and a start signal is supplied to the discharge lamp. After the lighting of the discharge lamp has been initiated, the drive frequency is defined that is higher, by a predetermined frequency displacement value .DELTA.F, than the drive frequency immediately before the discharge lamp is turned on. And the drive frequency for the switching device is shifted to a frequency area fb that is higher than a resonance frequency f2 when the discharge lamp is turned on.