When alternately applying an output voltage of a DC--DC converting circuit 3 to one discharge lamp, an output voltage on a non-load side in outputs having positive and negative polarities which is not applied to the discharge lamp during the light-up of the discharge lamp is restricted to be equal to an output voltage on a side which is applied to the discharge lamp. Moreover, when supplying a power to two discharge lamps while alternately switching a polarity of an output voltage through the DC--DC converting circuit 3, an output voltage on a non-load side in the outputs having positive and negative polarities which is not applied to the discharge lamp is restricted to be equal to the output voltage on the side which is applied to the discharge lamp if one of the discharge lamps is lighted up or an instruction for light-up is given and the other discharge lamp is lighted out or an instruction for light-out is given.