An image forming apparatus to execute user authentication, the image forming apparatus comprising: an input unit through which user information is input, wherein the user information includes a user identification and a password; an account management unit to manage an account information database included therein that stores user information corresponding to functions of the image forming apparatus; and a user authentication unit to execute user authentication based on the user information input through the input unit by using the account information database; characterised in that: the account management unit manages the account information database comprising a single user identification mapped to a plurality of passwords, wherein each said password is mapped to at least one respective function of the image forming apparatus; the user authentication unit executes the user authentication to activate the at least one respective function corresponding to the input user information when the input user identification and password match user information in the account information database; a display unit is arranged to display a user interface where the respective at least one function corresponding to the input user information is activated in response to a result of the authentication, whereby the user may request execution of the at least one function through the user interface; and the plurality of passwords includes a mandatory basic password that has to be set and at least one auxiliary password and when an auxiliary password is deleted through a user interface, the at least one function corresponding to the deleted auxiliary password is automatically changed to correspond to the basic password. an input unit through which user information is input, wherein the user information includes a user identification and a password; an account management unit to manage an account information database included therein that stores user information corresponding to functions of the image forming apparatus; and a user authentication unit to execute user authentication based on the user information input through the input unit by using the account information database; characterised in that: the account management unit manages the account information database comprising a single user identification mapped to a plurality of passwords, wherein each said password is mapped to at least one respective function of the image forming apparatus; the user authentication unit executes the user authentication to activate the at least one respective function corresponding to the input user information when the input user identification and password match user information in the account information database; a display unit is arranged to display a user interface where the respective at least one function corresponding to the input user information is activated in response to a result of the authentication, whereby the user may request execution of the at least one function through the user interface; and the plurality of passwords includes a mandatory basic password that has to be set and at least one auxiliary password and when an auxiliary password is deleted through a user interface, the at least one function corresponding to the deleted auxiliary password is automatically changed to correspond to the basic password. the account management unit manages the account information database comprising a single user identification mapped to a plurality of passwords, wherein each said password is mapped to at least one respective function of the image forming apparatus; the user authentication unit executes the user authentication to activate the at least one respective function corresponding to the input user information when the input user identification and password match user information in the account information database; a display unit is arranged to display a user interface where the respective at least one function corresponding to the input user information is activated in response to a result of the authentication, whereby the user may request execution of the at least one function through the user interface; and the plurality of passwords includes a mandatory basic password that has to be set and at least one auxiliary password and when an auxiliary password is deleted through a user interface, the at least one function corresponding to the deleted auxiliary password is automatically changed to correspond to the basic password.