PURPOSE:To reduce the storage time of a main transistor and a switching loss by inserting a parallel circuit comprising a capacitor and a resistor between the base circuit of the main transistor and the secondary winding of a current transformer. CONSTITUTION:When the base signal A of a transistor 12 is cut off at the time t3 and the transistor 12 is turned off, a flyback voltage is generated in a base winding 7 so that the base of a main transistor 4 becomes positive. When the main transistor 4 is turned on, the base current flows, and the capacitor 4 is charged to the voltage corresponding to the voltage drop across the resistor 15. Said charging current is flowed to the main transistor 4 as a reverse bias current and the reverse bias voltage is applied at the same time. In this way, the storage time of the main transistor 4 can be shortened as shown by the solid lines in the operating waveform diagrams (D) and (E).