A control server selects part of nodes included in a first communication network, generates a second communication network from the selected nodes, determines a forwarding probability of a packet between a node and its next hop node in the second communication network to calculate a communication volume between at least one pair of nodes included in the second communication network with respect to a prescribed traffic using the determined forwarding probability, and calculates a link cost for at least one pair of nodes included in the second communication network based on the determined forwarding probability and the calculated communication volume, adds at least one node included in the first communication network to the second communication network so that the calculated link cost satisfies a prescribed condition, and puts nodes not included in the second communication network in a low power consumption mode.