PURPOSE: To make it possible to detect a longitudinal flaw and the abnormality of a detector, by controlling automatic balance operation by comparing the detection output due to a differential amplifier and a reference value.

CONSTITUTION: The flaw detection signal due to a detector 1 is inputted to the differential amplifier 2 connected to an automatic balance circuit 4 for removing steady noise slow in change. When the output of this amplifier 2 reaches a reference level or more, an integration holding circuit 10 responds to the comparing output of a comparator 9 and the output of the circuit 4 is fixed so as to be held to the just-before value and the automatic balance operation to the amplifier 2 is not performed. By this mechanism, the output of the amplfier 2 is not brought to zero by automatic balance after the signal is abruptly changed by a long flaw or the abnormality of the detector and the longitudinal flaw or the abnormality of the detector is certainly detected.