an inverter device is provided , which carries out a switching action in response to a drive signal having a single frequency . an inductance element is connected in parallel with the output terminals of the device and capable of a parallel resonance with a total equivalent capacitance which is the sum of an equivalent electrostatic capacitance of the inverter device , as observed from the output terminals toward an input side , and an equivalent electrostatic capacitance of a load . with this arrangement , the charge and discharge loss due to the source - drain capacitance is reduced , and the efficiency is improved .