PURPOSE: To accurately detect an insufficiently insulated portion, while facilitating the works, by impressing two signals having different frequency compoenents upon a circuit and by connecting an AGC circuit in series with an amplifier which is operative to selectively amplify respective frequencies.

CONSTITUTION: The output terminal of a search coil 5 is connected with selective amplifiers 6a and 6b so that the obtained signals having the respective frequency components of an AC signal source 1 are amplified. The resultant output is fed to AGC circuits 9a and 9b. The AGC circuits are composed of negative feedback amplifying circuits so that the output to the next step can be held constant even if the spacing between a circuit 2 and the search coil 5 becomes larger than a preset level and if the outputs of the search coil 5 and the amplifiers 6a and 6b become smaller. The difference between the respective outputs of the AGC circuits 9a and 9b is calculated by a subtracting circuit 7, and the existence of the calculated output is displayed in a display device 8 so that the upstream and downstream portions of an insufficiently insulated portion of the circuit can be displayed. Thus, it is possible to generate the always stable detecting signals at the upstream and downstream portions.