To provide a VoIP authentication system having a high security level and capable of being easily operated.

An SIP server 106 is arranged in a network 105, and authentication data unique to a terminal are previously registered in this SIP server 106. In attempting to connect the network 105 via a network apparatus 104, a terminal 101 transmits its authentication data to the SIP server 106. The network apparatus 104 relays the authentication data for the SIP server 106. The SIP server 106 collates the authentication data, and notifies the terminal 101 of permission of connection if the authentication data are already registered. Based on this notification, the network apparatus 104 stores authenticated terminal data for which the SIP server 106 has permitted connection, and permits communication of the subsequent terminal 101 on the basis of this storage.