PURPOSE: To obtain an overcurrent detecting circuit operated stably by adding a constant voltage circuit using a Zener diode and a resistor to a comparison voltage circuit.

CONSTITUTION: A voltage value of voltage drop across a resistor 4 connected in series with a drain of a GaAsFET5 is used as a voltage value to be compared. The Zener diode 6 is connected in parallel with resistors 2, 3 connected in series and a resistor 7 for current limit is connected to a negative power supply (-B) for gate bias. Then the voltage drop by the resistor 2 is constant at all times and even if the voltage for drain positive power supply 10 is low, the voltage of the resistor 4 to be compared, that is, the overcurrent detecting level is unchanged.