PURPOSE: To restore a node for control from a faulty state without stopping a system by doubling the node for control in such a state that management information can be transferred between the two nodes.

CONSTITUTION: Monitoring packets are sent from a currently used node 100a for control at regular time intervals. Each node writes the information of a fault detected at its own node in the status bit of the packets at the packet arriving timing. When the adjacent node or adjacent ring transmission line in the forward direction becomes faulty, the currently used node 100a makes communication by only using monitoring packets arrived from the normal ring on the other side so that the said monitoring packet can travel round to the node 100a. Since packets with instructions regarding the occurrence and restoration of faults in their command bit are sent to relevant nodes after the status information of communicable nodes is collected by means of the packets in such way, control can be made in accordance with a fault restoring procedure.