The present invention provides a repeater which automatically closes a dynamically opened port in accordance with prior setting, the repeater including: a port mapping table where a global IP address and an external port number, or an external port number on the Internet is associated with a local IP address and an internal port number of a terminal connected to a LAN; a control unit which, receiving a communications packet to which an external port number is specified, converts the external port number to an internal port number based on the port mapping table and transfers the internal port number to the LAN; a timer unit which counts the unoccupied time of the port after transferring a communications packet to which an external port number is specified; and a port management unit which checks whether the registered machine is present on the LAN and in case it is absent, deletes registration concerning the external port number from the port mapping table.