When a switching element (33) changes from ON state into OFF state, a capacitor (35) is charged. The rising of the voltage of the switching element becomes dull relative to the falling of the current flowing in the switching element. When the switching element changes from OFF state into ON state, because a diode (34) has been in ON state, switching of the switching element is zero-voltage switching. When an auxiliary switching element (39) changes from ON state into OFF state, an auxiliary capacitor (41) is charged. The rising of the voltage of the auxiliary switching element becomes dull relative to the failing of the current flowing in the auxiliary switching element. When the auxiliary switching element changes from OFF state into ON state, because an auxiliary diode (40) has been in ON state, switching of the auxiliary switching element is zero-voltage switching. Thus the switching loss is reduced