PURPOSE: To certainly detect the discontinuation of optical signals when an optical-fiber transmission line is cut off by synchronizing the frame synchronization circuit of the 2nd terminal station by means of the frame signal of the 1st terminal station and the frame synchronization circuit of the 1st terminal station by means of the frame signal of the 2nd terminal station which is different from that of the 1st terminal station.

CONSTITUTION: When an optical-fiber transmission line 3 is cut off, signals 102 to which code rule contravention is added by means of a code conversion circuit 11 in accordance with the frame pattern of a frame signal generation circuit 10 and sent from a terminal station 1 through an electrooptical converting circuit 12 and directional coupler 13 are reflected by the cut face of the line 3 and returned to the station 1. The signals returned to the station 1 are inputted to a frame synchronization circuit 16 as a code rule contravening pulse train 108 through the coupler 13, a photoelectric conversion circuit 14, and a decoding conversion circuit 15. Since the pulse train is synchronized to the frame pattern of a frame signal generation circuit 20, but not to other frame patterns, the circuit 16 generates an optical signal discontinuation alarm 107. Similarly, an optical signal discontinuation alarm 207 is generated from the frame synchronization circuit 26 of another terminal station 2.