To facilitate authentication even when the number of registered users increases by allowing an index server to retrieve the index of an authentication request and determine one of a plurality of authentication servers, and making the determined authentication server to authenticate the authentication request.

The index server 240 is connected to a network and receives user authentication requests from network access servers 1, 2, 3, 4, and 5. Authentication servers 220 and 223 are connected to the index server 240 and the index server 240 determines an authentication server to perform authentication between the authentication servers 220 and 223. Then the authentication requests received from the network access servers 1, 2, 3, 4, and 5 are transferred to the determined authentication server. Consequently, the authentication can be facilitated even when the number of registered users increases.