To improve the versatility and convenience of a user authentication system which can restrain the unauthorized use of a service-providing device without specifically limiting usable service-providing devices.

In a user authentication system 1, a mail server 5 authenticates whether a mail account and a password, which are inputted in an image-forming apparatus 3, accord with the one of a plurality of mail accounts or the like. A charging host 7 holds a plurality of mail accounts or the like which are preliminarily registered as persons who are permitted to use the image-forming apparatus 3, and authenticates whether an inputted mail account accords with one of the mail accounts of a plurality of preliminarily registered users when authentication by a first authentication means is successful, and permits the users, who input the mail accounts or the like, to use the image-forming apparatus 3 when the authentication is successful.