A first energy ratio A/B of a first alternating current of a steady frequency supplied to a lamp is set to a value C, when the lamp is horizontally placed. A second alternating current of a lower frequency, whose second energy ratio A/B is set to the value C, is inserted. When the lamp is vertically placed, a first energy ratio A′/B′ is set to the value C or a value D, which is smaller than the value C. A second ratio A′/B′ is set to the value D or a value E, which is lower than the value C. A and A′ each represents an energy that flows from a first electrode of a pair of electrodes of the lamp to a second electrode of the pair. B and B′ each represents an energy that flows from the second electrode to the first electrode.