To provide an authentication method which can be used irrespective of a receiving status of an electric wave of a personal digital assistant and can perform authentication a plurality of times in a short time.

This authentication system comprises: an information terminal equipped with an online service server and a noncontact IC chip reader/writer; and a personal digital assistant which is equipped with an authentication server, a noncontact IC chip and an authentication client and generates authentication information. This system is provided with: a means wherein an information terminal performs need determination of a terminal user authentication and determines whether the authentication is needed or not, performs authentication of terminal user by executing a terminal user authentication means, generates when the determination is success, authentication information based on the information read from the noncontact IC chip, writes generated authentication information in the noncontact IC chip, and ends an authentication client; and a means wherein the information terminal transmits the authentication information to the online service server, while reading authentication information from the noncontact IC chip through the noncontact IC reader/writer.

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