It is possible to appropriately execute packet filtering and unauthorized access detection in a condition where communication using a mobile network is operated. A home agent (404A) acquires the IP address, port number, medium type obtained by call control of a call control relay server (403A) and reports them to a firewall (700). The firewall (700) has a packet filtering device (400) which performs packet filtering so that an IP packet specified by the set of the IP address and the port number is passed. Moreover, an unauthorized access monitoring device (701) acquires information on the corresponding medium type from the normal access judgment conditions predefined for each of the medium types. According to the normal access judgment condition, packet monitoring is performed and the onedeviating from the normal access is detected as an unauthorized access.