PURPOSE: To improve the reliability of a system by providing an optical switch for transmission-line switching between an optical transmitter-receiver and an optical branching and mixing device which are constituent elements of each terminal station.
CONSTITUTION: At a terminal station A of multiple constitution, an optical switch 8a is provided between optical transmitters 1 and 2, and an optical branching device 3 having an optical mixing function. At a terminal station B, an optical switch 8b is also provided between optical transmitters 1 and 2, and an optical mixing device 3 having an optical mixing functon. If a fault occurs to either one of optical fibers 4 and 7 as an optica transmission line, the device is so constituted that the transmission-line is switched to the other optical fiber side on whch a fault is not generated.
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