PURPOSE: To attain always stable detection by extracting a detected signal current from an output side of a 2nd current mirror circuit.

CONSTITUTION: Let a direction of a current flowing from a power supply Vcc to ground GND be positive polarity, currents of DC current sources 3, 4 be I1, I2 respectively, a current of a signal current source 2 be is, an input side drive current of a 1st current mirror circuit 1 and an output side load current be i1, i2 respectively, an input side drive current of a 2nd current mirror circuit 5 and an output side load current be i3, i4 respectively, and the currents flow in the direction of the arrow shown in Figure. A difference current of the currents i1, I2 in either the positive or the negative cycle is extracted from the output of a 2nd current mirror circuit 5 as a load current. Thus, even when a temperature or a process condition is changed, stable detection is attained.