Access routers (100-1 and 100-2) perform packet transfer using an IP address and a port number between a global network allowing mutual access using the IP address and a network other than the global network. Port resource assigning device (400) manages the port number in units of IP addresses, and, when performing the packet transfer, assigns the port number managed by means of the IP address preliminarily provided for access routers (100-1 and 100-2). Access routers (100-1 and 100-2) perform packet transfer using the IP address preliminarily provided for access router (100-1 and 100-2) and the port number assigned by port resource assigning device (400).