A user authentication method is applied to a user authentication apparatus for authentication using a unique ID that is information unique to a communication apparatus. The method includes acquiring a user ID as information identifying a user, and detecting a unique ID corresponding to the user ID from a user ID list. The method also includes making a call with a predetermined number of rings to a communication apparatus having the unique ID. Additionally, the method includes receiving a callback made by a communication apparatus after a predetermined number of rings of the call in response to the call. In addition, the method includes determining whether a unique ID of the communication apparatus having made the callback matches the unique ID of the communication apparatus to which the call has been made, and determining, if there is a match, that a user having the user ID is an authorized user.