To make a communication cost dynamically adaptable to the variation of the state of a communication line.

An inspection signal processing section 14 outputs an inspection signal and acquires response signals from communication lines E1, E3 and E5 thus grasping the state of the communication line. If the state of the communication line thus acquired is changed from that stored on a cost table 16a, a cost calculating section 14b calculates a new communication cost for the communication line based on the changed state of the communication line and delivers the calculated communication cost to the cost table 16a thus updating the cost table 16a. Information of the newly calculated communication cost are delivered to nodes 2-4. Alternatively, a node 1 acquires the communication cost information of communication lines E2, E4, E6 and E7 inputted from one of the nodes 2-4 and updates the cost table 16a constantly thus grasping the communication cost of the communication line in the entire network A dynamically.