A method and apparatus for authenticating a user is disclosed that includes measuring biometric information of the user to create biometric measurement information, determining whether a private key included in a user authentication request signal matches a private key issued in advance to the user, comparing pre-set biometric authentication information for the user with the biometric measurement information, calculating a matching ratio when a match is detected, authenticating the user having provided the biometric information as an authorized user based on a result of comparison of the calculated matching ratio with a pre-determined biometric authentication threshold value, and providing an updated private key to the information processing device based on a result of comparison of the calculated matching ratio with a pre-determined updated threshold value.