COMMUNICATION NETWORK SYSTEM

PROBLEM TO BE SOLVED: To maintain normal communication between rings even when the switching of a duplexed line spread between inter-ring connection node devices is failed between ring networks or any fault of the inter-ring connection node device itself occurs. SOLUTION: Concerning this communication network system, ring networks A and B are provided with two inter-ring connection node devices 113A, 114A, 113B and 114B, a control node device 110A controls the devices 113A and 114A while switching them and a control node device 110B controls the devices 113B and 114B while switching them as active or reserve systems. Even when an inter-ring transmission line 31 between the devices 113A and 113B operated as the active system is disconnected, the devices 114A and 114B set on standby as the reserve system are activated as the active system and while utilizing an inter-ring communication line 41 between these devices 114A and 114B, communication can be maintained.