PURPOSE: To prevent the breakdown of a main transistor, by connecting a Zener diode and a transistor between the base and the emitter of the main transistor connected with a load which generates the spike voltage when a power supply is turned on and off.

CONSTITUTION: A main transistor TR11 is turned on or off when the input signal given from a terminal 10 has a rise or is broken. Thus the spike voltage is generated by a reactance load 12. This spike voltage exceeds the specific value that is decided by the voltage VBES obtained between the base B and the emitter E of a protecting TR18 connected between the B and the E of the TR11, a resistance 20 and the Zener voltage VZ of a Zener diode 19. In such a case, a working command is given to the base of the TR18. Thus the TR18 conducts to absorb the spike voltage. In this case, the current flowing to a diode 19 is set at 1/hFE(hFE: current amplification factor of TR11), and the capacity can be reduced for the diode 19. Thus the breakdown and the deterioration of characteristics can be avoided for the TR11 with inexpensive constitution.