Unexpected call disconnection during normal time is prevented. When a gateway device 1 installed on a POI border receives RTP packets even though reception of RTCP packets is stopped, the gateway device 1 generates RTCP packets and sends out the RTCP packets to the gateway device 1's own network side or the gateway device 1 generates a call control signal showing that media transfer is continued and sends out the call control signal to the gateway device 1's own network side. Thereby, even in the case of performing interruption monitoring of RTCP packets within the gateway device 1's own network, it is possible to prevent unexpected call disconnection during normal time accompanying change in RTCP packet sending-out conditions.