PURPOSE: To enable to retrieve the degree of abnormity and failure of each relay unit, by detecting the output of electric-photo conversion, delivering the response signal of specific pattern via FF and BPF, and monitoring the waveform of the signal at each terminal station.

CONSTITUTION: The photo electric conversion element 15 receives the specific pattern signal for failure retrieval delivered from the terminal station 11, and the optical relay retrieval for the circuit which converts electric into light 21 and delivers it to the terminal station 12 via the optical relay unit 10, by adding the signal processing such as amplifications 16, 17, timing pick up 18 and identification and reproduction 19 to the output. That is, the output is the conversion circuit 21 is branched, photo detection 32 is made, the response signal of specific pattern is obtained via FF34 and BPF 35, and it is delivered to the terminal stations 11 and 12, and the failure of waveform is monitored at each station. Thus, the abnormity of each relay unit 10 and the degree of failure can be retrieved.