A user authentication processing is performed and an authentication session ID is returned to a terminal 500 (A14). An authentication server 600 issues and stores an authentication ticket (A17). The authentication ticket and authentication session are returned to the terminal 500 (A18). A user 100 transmits a request of service provision and the authentication ticket to a service provider's server 700, and the service provider's server 700 transmits the authentication ticket to the authentication server 600 (A20). The authentication server 600 performs an authentication processing of the authentication ticket (A21), and the authentication result is notified (A22). In the case of the authentication approval, a service session ID is issued together with the notification of authorization (A23). When receiving the notification of the authentication approval, the terminal 500 performs an establishment processing of the session using the received service session ID, and stores the service session ID (A27).