Authentication method, authentication system and the authentication server

User authentication process is executed, the authentication session ID is returned to the terminal 500 (A14). Authentication server 600 issues and holding an authentication ticket (A17). Authentication ticket and return the authentication session to the terminal 500 (A18). User 100 sends a request to the authentication ticket of the service provided to the service provider server 700, service provider server 700 sends an authentication ticket to the authentication server 600 (A20). The authentication server 600 performs an authentication process of the authentication ticket (A21), the authentication result is notified (A22). If authentication authorization, the service session ID is issued along with the notification of the permission (A23). When authentication permission notification is received, it performs establishment processing session using the service session ID terminal 500 receives and holds the service session ID (A27).