To solve the problem wherein the reliability of extra traffic having low priority is low because the extra traffic is disconnected when by-pass operation is performed in service traffic having high priority.

A transmission/communication system where a plurality of transmission devices are connected by a transmission line of duplex structure having an active transmission line and a stand-by transmission line and a service communication pass having high priority and an extra communication pass having low priority are set refers to the set state of the communication pass on a route for evading a fault section when detecting the fault in the transmission line and by-passes the extra communication pass in which a transmission area corresponding to a transmission area being used by the extra communication pass at present is free on the route for evading the fault section to the transmission area concerned.