Abstract: An object of the present invention is to provide a user authentication method and system, an information terminal device and a service providing server, a subject identification method and system, a correspondence confirmation method and system, an object confirmation method and system, which are capable of assuring authentication and/or confirmation of a person, an animal and a plant, or an object and improving precision thereof, and program products for them. In order to perform user authentication when a user of an information terminal device 30 such as a cellular phone or the like receives a service from a service providing server 20 via a network, iris authentication is performed, and a password is automatically updated every time a service is provided to generate a new password to be used when a next service is provided, and the new password is stored in both the information terminal device 30 and the service providing server 20.