A radio base station includes a mobile communication system that sends and receives control signals between a mobile station and a mobile switching center via a first radio bearer, a second radio bearer, and a wired bearer. The relay node includes a first radio bearer function, a second radio bearer function, an IP layer function, an SCTP layer function, and an application layer function. The radio base station includes a second radio bearer function, a wired bearer function, and an IP layer function. The mobile switching center includes a wired bearer function, IP layer function, SCTP layer function, and an application layer function. Keep-alive processing is performed between the SCTP layer function of the relay node and the SCTP layer function of the mobile switching center. Security processing is performed between the IP layer function of the relay node and the IP layer function of the mobile switching center.