To protect the circuitry of a current resonance power supply by turning a switching element on/off if a commutation current flows through a commutation diode connected in reverse parallel with the switching element when switching frequency is varied abruptly thereby preventing a through current from flowing into the switching element.

In a current resonance converter, an I/V conversion circuit 51 and a current state detection circuit 52 detect the state where a commutation current flows through commutation diodes D1 and D2 connected in reverse parallel with switching elements Q1 and Q2 as the state of a current flowing to the primary side through switching operation. When a commutation current is flowing through the commutation diodes D1 and D2, a drive signal waveform for not turning the switching elements Q1 and Q2 on/off is obtained based on the detection results.