Even in a high pressure discharge lamp causing temperature difference between electrodes when an AC current is supplied, the temperature difference is eliminated to suppress the arc movement, thereby suppressing flickering caused thereby during stable lighting. The series resonance circuit includes a first inductor, a first capacitor and a variable capacitance diode which are connected together in series. This desired subset of slaves ten can be moved to one of the states that is unaffected by commands that cause the selection of other desirable subsets of slaves.