To assign an address of each server freely in each closed network to utilize the server and to apply customized common service to each closed network without losing closing performance of each closed network.

A host 1 (H5) requests connection to a server 26 by using a local address assigned in advance to the server 26. A root side NAT 30 authenticates a network to which a host 1 (H5) belongs by a network identifier assigned to each network. Then a local address assigned in advance to the host 1 (H5) is converted into prescribed global address depending on the network to be authenticated and the local address assigned to the server 26 is converted into the global address assigned in advance to the server 26.