The present invention improves the reliability and safety of communication in a communications system wherein communication is relayed. When an on-premise terminal 100 sets relay information, such as NAT information or the setting information of a packet filter, in a gateway 110, the relay information is set after confirming whether the NAT information for which a setting attempt is being made duplicates the NAT information of another on-premise terminal 101. The same applies to the case of deleting relay information. If the NAT information is duplicate, then interference with the communication of the other on-premise terminal 101 is prevented by modifying the relay information for which a setting attempt is being made, specifically the port number of the gateway 110 on a global network 001.