PURPOSE: To reduce switching loss of a switching transistor and to realize high efficiency by providing first and second current detecting means, means for adding outputs from the first and second current detecting means, and a circuit for turning the switching transistor OFF.

CONSTITUTION: When an NPN transistor Q1 is turned OFF and no current flows from a resonance capacitor C1 to a second current detecting resistor R2, a PNP transistor Q2 is turned OFF. Consequently, emitter-base voltage VEB2 of the PNP transistor Q2 can be represented by a formula VEB2=VR1+ VR2+VFD2-VR5. where VR1, VR2 and VR5 are voltages across resistors R1, R2, R5 and VFD2 is forward voltage drop of a diode D2. When the NPN transistor Q1 is turned OFF and no current flows from the resonance capacitor C1 to the second current detecting resistor R2, VR1+VR2 is very low and VEB2 is also low.