To restrict actions such as spoofing and thereby prevent tapping and leakages of data by certifying whether or not each communication device such as a storage device on a communication line is to be connected to the communication line.Upon receipt of a packet that contains an IP address in its IP header and stores a certificate in its certificate payload from a storage device 300, an authentication device 200 compares an IP address that is recorded in the certificate and the IP address that is recorded on the IP header of the packet. If the comparison results in a match of these IP addresses, the authentication device 200 can certify that the storage device 300 is a device for which a certificate issuing device 100 has properly issued the certificate.