PURPOSE: To protect a transistor (TR) from a large displacement current, by operating a switching device so as to short-circuit the base and emitter, when a voltage change detector detects the voltage change applied between the collector and emitter of the TR.

CONSTITUTION: A base drive device 2 drives a TR1. The voltage change applied to the collector and emitter of the TR1 is detected with a differentiating circuit comprising resistors R10, R13 and a capacitor C11. A detected current flowing to the differentiating circuit charges up the C11 and the charging current is branched into the R13 and the base of a TR12. Thus, the TR12 is turned on and the base and emitter of the TR1 is short-circuited. The displacement current because of the voltage change between the emitter and collector of the TR1 is led to the base, allowing to prevent the destruction of the TR1.