A primary winding 5a of a transformer T and a switch Q1 are connected in series with a DC power source Vdc1. A series circuit composed of a switch Q2 and a snubber capacitor C3 is connected in parallel with the primary winding or the switch Q1. A saturable reactor SL1 is connected in parallel with the primary winding 5a. A series circuit composed of a diode D1 and a capacitor C4 is connected in parallel with a secondary winding 5b of the transformer T, to form a rectifying/smoothing circuit. A control circuit 10 turns on and off the switches Q1 and Q2 alternately and turns off the switch Q2 when a current to the switch Q2 increases.