First and second switch are connected in series to both terminals of a DC power source. A series circuit comprising a reactor, a primary winding of a transformer, and a capacitor connected in series, and is connected to a node between the first and second switches and one terminal of the DC power source. A rectifier smoothing circuit rectifies and smoothens a voltage generated across a secondary winding of the transformer and outputs a DC voltage. A control circuit alternately turns the first and second switches on and off. A voltage detection circuit detects the DC voltage from the rectifier smoothing circuit. A signal generation circuit generates a feedback signal from the DC voltage detected by the voltage detection circuit, and outputs the signal for turning the first and second switches on and off. A load current detection circuit detects load current contained in resonance current flowing through the capacitor.