Abstract: A small size output transformer is capable of stablely producing sine wave oscillation at high efficiency. A drive circuit including a switching circuit is connected to DC power source input units and output transformer is connected to the drive circuit. At the drive circuit, the switching circuit is turned on and off based on a feedback signal of a primary side resonance voltage induced in an input side primary winding of the output transformer. The energizing direction of the DC power source to the output transformer is changed in direction by the on and off of the switching circuit, and the power source is applied as an AC signal to the input side primary winding of the output transformer. A phase detecting circuit detects the phase of the resonance wave form at the primary side of the output transformer sand outputs the phase signal.