A cylindrical electrophotoreceptor is disclosed. The detecting apparatus thus displays an insertion shape of the endoscope at a timing as needed. For example, if Va is within a normal range and moreover at least one of the individual output voltages is not within a normal range, it is diagnosed that water blocks the fuel electrode. A hook or clamp is connected to the hydraulic jack for grasping the stressed cable in order to apply the force thereto, and a dial gauge is mounted onto the frame for measuring the deflection of the stressed cable once the force has been applied. A second signal generator connected with the second input of the comparator is operative to generate the second signal as a reference voltage defining a prescribed impedance mismatch threshold associated with the circuit to be monitored. The power-source passages, too, are formed independently of other digital circuits.