To authenticate the legality of a user at a lower cost with a high accuracy by using the positional information of a portable telephone.

An authentication server 3 has an authentication storage means for storing the positional information in a time series of the portable telephone 1, a receiving means for receiving a log-in requirement from a terminal 2, and a computing means for computing the current positional area of the portable telephone 1 on the basis of the positional information in the time series stored in the authentication storage means. The authentication server 3 further has an authentication means for receiving a user area specified as the presence place of the user from the terminal, and authenticating the log-in requirement from a legal user when the user area and the current-place area coincide.

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