It is made possible to appropriately execute packet filtering and unauthorized access detection in a situation in which mobile network communications are operated.A home agent 404A acquires IP address, port number, and media type obtained by call control of a call control proxy server 403A and sends them to a firewall 700. A packet filtering unit 400 of the firewall 700 performs packet filtering so as to allow the IP packet determined according to the IP address and port number pair to pass through. An unauthorized access monitor 701 takes out information of the corresponding media type out of the normal access determination condition previously defined for each media type, monitors a packet based on the normal access determination condition, and detects that access deviating from normal access is unauthorized access.