PURPOSE:To recognize it from an opposite station that an optical transmission line from a counter station is normal by interrupting the transmission of an optical signal at an interval after optical input signal interruption information is transmitted at the station detecting an open line fault. CONSTITUTION:If a broke-down fault of an optical fiber takes place at a point F of an outgoing line 15, then an optical input signal interruption detection circuit of an optical signal-electric conversion circuit 5a in an optical transmitter 1a detects the interruption of input, a specific address pattern generated in an address pattern generating circuit 9a is transmitted to an optical transmitter 1b of an opposite station via a control circuit 8a through an optical signal output port 13a and an incoming line 14, and the optical signal output from the optical signal output port l3a is interrupted at an interval. When an address pattern detection circuit 10b of the optical transmitter 1b of the opposite station detects the address pattern, the normality of the incoming line 14 is transferred to a display circuit 11b and displayed, and the input interruption is detected by an optical input signal interruption detecting circuit of an optical signal- electric conversion circuit 5b and the optical signal output from an optical signal output port 13b is interrupted.
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