A method and apparatus for user authentication using infrared communication of a mobile terminal is disclosed. The user authentication system, which is for identifying a user who requests service from an automated machine, comprises a mobile terminal for transmitting electronic signature data for the user in the form of an infrared signal, automated information providing means for providing the requested service after verifying the validity of the electronic signature data, and certificate providing means for registering an authorization certificate in response to a request from the mobile terminal and transmitting the certificate to the automated information providing means for verifying the user authentication.