Upon receipt of a request to connect to an ad-hoc network from an external communication node, an address assigning unit obtains information on an IP address assigned to the external communication node. The address assigning unit determines whether the IP address is already used by any of communication nodes that are connected to the ad-hoc network based on network management information stored in a storage unit. The address assigning unit generates a new IP address that is used none of the communication nodes when the IP address is already used, and transmits the new IP address to the external communication node. The external communication node is assigned the new IP address, and connects to the ad-hoc network.