PURPOSE: To prevent surely the retrigger of a transistor (TR) to be driven attended with resonance by providing a PNPTR for forward bias current supply and a diode for resonance blocking to a driving power supply.

CONSTITUTION: A control signal generator 7 is connected to a PNPTR14 via a signal inverting device 15 and a limit resistor 10 and to a base of a TR4 via a limit resistor 11 respectively, and a diode D13 for blocking resonance is provided to the circuit for the TR4 supplying reverse bias current. As a result, even when a potential at a point B of an output terminal of a drive circuit 12 is lower than the point A just after the TR1 to be driven is interrupted, a forward bias voltage is not applied to the TR14 for supplying forward bias base current and the TR14 keeps the interrupting state. Further, even if the potential at the point B is lower than the potential of a reverse bias drive power supply 6, no forward bias current is applied to the TR1.