PURPOSE: To extend the network while utilizing the survivability of a self-healing ring.

CONSTITUTION: An additional node 6 is connected to a ring network 10 comprising nodes 1-5 each having a loopback function to form an interlocking network. A service selector 1a of the node 1 selects either of signals from the node 2 or the additional node 6 and provides the selected signal to the node 5 as an output. A path selector 6b of the additional node 6 selects either of the signals sent from the nodes 1, 2 and provides an output to the outside of the ring network 10. The service selector 1a and the path selector 6b have a monitor control function to select a normal signal in the two input signals at all times and decide the triggering for changeover and the switching direction in an autonomous dispersion way.