PURPOSE: To detect the damage of a conductive film and a conductive band by providing resistors between a power supplying circuit and electrodes and connecting at least one of the circuits across electrodes separated near the center and adjacent to each other to a detecting sensor and a high-frequency coil.

CONSTITUTION: When a conductive band 4b is broken, all current flows from an electrode 3a to electrodes 3c and 3d and, therefore, more current flows at the electrode 3c than at the electrode 3d. Also, since a potential difference is produced at the electrodes 3c and 3d by resistors 7 and 8 and a current flows through a detecting sensor 5 to compensate for this, the transformer of the detecting sensor 5 causes magnetic saturation and the impedance of a connected transformer from an transmitter circuit to a B.P.F circuit lowers. If it is broken at a point apart from the center of the conductive film 2, a slight current will flow from an electrode 3b to the electrode 3a and from the electrode 3c to the electrode 3d. Therefore the transformer of the detecting sensor 5 is saturated magnetically and the output of a detecting circuit 9 becomes '1' to detect error.