An authentication system includes an authentication information input device to which a user inputs authentication information, an authentication server, and at least one image forming apparatus, all connected via a network. The authentication server acquires the authentication information input via the authentication information input device and performs user authentication based on the authentication information. The authentication server includes a transmitter to transmit a result of the user authentication indicating whether or not the user authentication is successful and information of the user identified by the authentication information to at least one of the image forming apparatuses. The image forming apparatus includes a receiver to receive the result of the user authentication and the user information transmitted from the authentication server, and an apparatus controller to switch the image forming apparatus from an unusable state to a usable state according to the result of the user authentication and the user information.