A serial resonance circuit is prepared. The serial resonance circuit is configured of a pair of resonance circuit is configured of a pair of resonance elements serially connected to each other at a midpoint therebetween and exhibiting resonance frequencies equal to each other. To both ends of the serial resonance circuit, phase shift circuits that shift phases so that the phases are reverse to each other are connected. A resonance output obtained from the serial resonance circuit when AC power is supplied between the midpoint and an input side of the phase shift circuit is obtained as an external output.