While supervision is carried out, in this case, by making the delay time of the delay element 42 4 digits, it is impossible to supervise for a period during which, as shown in 65 of FIG. 5, a part of the signals and the synchronous pulses Y return to the input side in the examples of FIG. 1 in which signals other than input signals are superposed on the output side and FIG. 5, respectively.