To provide an authentication system allowing a client terminal to safely be provided with on-line service from an on-line service server.

This authentication system has: the on-line service server; an information terminal device to be provided with the on-line service; an OTP (One Time Password) server performing processing related to authentication of transaction contents in the on-line service and login authentication of the information terminal device; and mobile terminal device displaying OTPs used for the authentication of the transaction contents and the login authentication. The mobile terminal device has a means separately transmitting acquisition request of the transaction contents-authenticating OTP and the login-authenticating OTP necessary when the information terminal device receives the on-line service from the on-line service server, to the OTP server, receiving the transaction contents-authenticating OTP and the login-authenticating OTP from the OTP server, and displaying them.

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