PURPOSE: To surely prevent overcurrent occurring by varying the voltage of the base terminal of a bipolar transistor by operating a photocoupler with the voltage drop of a resistor according to the overcurrent of the FET of an amplifier element, and controlling a current between the collector and emitter of the bipolar transistor.

CONSTITUTION: When the overcurrent flows from a positive power source supply terminal 12 to the drian of the FET 1 comprising a high frequency power amplifier due to the fault of the FET, large voltage drop occurs at the resistor 4, and the current flows on the diode part 7 of the photocoupler 6, which emits light, and a transistor part 8 is turned on, and the current flows from the positive power source supply terminal 12 via the transistor part 8 and the resistor 10, and the voltage equivalent to the voltage drop of the resistor 10 is applied to the base terminal of a PNP bipolar transistor 5 passing the resistor 9. The resistors 9 and 10 are set so as to permit the base current of the transistor 5 to flow as the ordinary current in use of the FET 1, however, the potential of the connecting point of the resistors 9 and 10 is increased by setting the transistor part 8 at an energized state, and the current flowing between the collector and emitter of the transistor 5 can be suppressed, which prevents the overcurrent occurring.