PURPOSE: To improve the using efficiency of a network by executing communication with priority when the communication is required, and when no communication is applifed from a node for a fixed time or more, judging the disconnection of the node from the network and updating information in a routing table.

CONSTITUTION: When no message is transferred, respective nodes N0 to N6 send the dummy packets of connection information to a router R in each fixed period or within a predetermined fixed time. When communication is requested, the dummy packet transmission is suppressed to execute the communication with priority and the router R monitors the packets transferred on a transmission line. When communication or dummy pakets are not broadcasted from the nodes N0 to N6 for the previously fixed time or more, the router R judges the disconnection of the nodes from the network L, broadcasts the connection information to respective nodes N0 to N6 and allows respective nodes N0 to N6 to update the routing inforamtion.