PURPOSE: To accomplish a quick and accurate detectin of current peak value while minimizing effect due to temperature changes and secular changes in the circuit elements.

CONSTITUTION: When current to be measured is limited, the voltage across a resistance 11 is so low that a silicon bidirectional switching element (BDT) 21 is off and it is applied between terminals of the BDT21. As the voltage applied to the BDT21 exceeds a switching voltage with the increase in the current being, the BDT21 conducts causing the current to pass mostly through a variable resistor 12, a resitance 13 the BDT21 and a rectifier bridge 16. And the output of the bridge 16 operates a photocoupler 17 regardless of the polarity of the current to be measured. This enables on/off operation of the BDT with a limited variation in charcteristics thereby reducing effect due to temperature changes and secular changes thereof.