When a client has accessed a server via routers, the server inherits an IP address used during the access and further adds an arbitrary system ID thereto, thereby generating a new connection address. The server then notifies the system ID to the client and further generates a communication process for communicating with the client. The communication between the client and the server from then on is performed, based on the new connection address, by the communication process. Since the connection address, which is generated anew for the communication between the communication process of the server and the client, includes the same IP address as was used during the initial access, the NAT functions of the routers can be used without any problems.