An output terminal of a positive voltage generating circuit and an output terminal of a negative voltage generating circuit are connected in series, and an output terminal of the negative voltage generating circuit is connected to a ground via a resistor. A switching circuit and a series connection circuit of resistors are connected in parallel to each other between the output terminals of the positive voltage generating circuit and the negative voltage generating circuit. The switching circuit is on-off driven by a voltage signal taken from a junction point between the resistors, and the switching circuit is on-off driven by a voltage signal taken from a junction point between the resistors. Accordingly, when the polarity of an output voltage is switched, electric charges accumulated up to that point are discharged through the switching circuit of a voltage OFF-side polarity, so that the voltage quickly falls.