In a gateway device for carrying out a data relaying at a transport or upper layer between a first terminal device and a second terminal device which are capable of carrying out communications through networks, information regarding a security association set up between the first terminal device and the second terminal device in order to carry out communications with guaranteed data secrecy or in order to carry out communications with guaranteed data authenticity managed, and the decryption/encryption processing or the authentication processing is carried out according to the managed information regarding the security association along with the data relaying at the transport or upper layer.