A series circuit of a first main transistor 4 and a first main diode 6 is connected across a DC input voltage source 1, and a filter capacitor 13 and a series circuit of a second main diode 12 and a second main transistor 8 are connected in parallel with the output load side. A filter reactor 7 is connected between the juncture of the first main transistor 4 and the first main diode 6 in the series circuit and the juncture between the second main diode 12 and the second main transistor 8 in the series circuit. Resonant capacitors 5 and 9 in effect are connected across the collector-emitter path of the first and second main transistors 4 and 8, respectively. In operations other than that with small load current, when the first and second main transistors 4 and 8 are turned off, the collector currents therefrom are caused to flow to the resonant capacitors 5 and 9, respectively, thus obtaining soft switching. In the small load operation with a pulse width less than that in the neighborhood of the resonant time, instead of the on-off operation of the first or second main transistor, hard switching is made with a first auxiliary transistor 2 at the step-down time and with a second auxiliary transistor 11 at the step-down time.