Relay server, relay server service management method, service providing system, and program

An application service delivery method capable of making use of an existing SIP server function and easily carrying out authentication and billing. When a user who operates a client requests a service delivered by an application server, the present method establishes an SIP session between a client and application server using a standard SIP procedure via a relay server, and provides a direct service between the client and application server after establishment of the SIP session. The relay server carries out user authentication in the stage of establishment of the SIP session, which eliminates the necessity for the authentication function of the application server and makes it possible to easily construct the application server.