PURPOSE: To prevent secondary destruction of a power transistor (TR) by increasing the rate of fluctuation of an emitter current against the fluctuation of a voltage in a region where a voltage impressed between a collector and emitter is large.

CONSTITUTION: A series circuit comprising the 2nd Zener diode 9 and a resistor 8 is connected in parallel with a resistor 3 being a component of a protection circuit in a circuit device where the destruction of the power TR1 is prevented by using a Zener diode 6. Through the constitution above, a characteristic with a slope θ is adopted so as not to lose the circuit characteristic at a region where an impressed voltage VA between the collector and emitter of the power TR1 is comparatively small, while the rate of fluctuation of the current against the voltage fluctuation is increased at a region where the impressed voltage is larger. As a result, the secondary destruction of the power TR1 is prevented in advance.