A disclosed image forming apparatus includes an authentication information unit including login information of a user and an authentication key; an authentication key providing unit including the authentication key; functional units; and a functional-unit authentication unit including authentication information of the functional units and a first verification unit configured to determine whether the functional units are authenticated. Each of the functional units includes an authentication key obtaining unit for obtaining the authentication key from the authentication key providing unit if the first verification unit determines that the functional unit is authenticated. The authentication information unit further includes a second verification unit for determining whether the authentication key obtained by the authentication key obtaining unit matches the authentication key in the authentication information unit, and a login information providing unit for providing the login information to the functional unit if the second verification unit determines that the authentication keys match.