An image forming apparatus includes an user authentication part 31 for executing authentication of a user, a function enabling part 32 for enabling at least one function available for the first user based on authority of the first user set in advance when authentication of the first user executed by the user authentication part 31 results in success, and a function extending part 33 for extending at least one function available for the first user to enable when authentication of the second user executed by the user authentication part 31 results in success with at least one function available for the first user being enabled. Therefore, with a user who is not authorized to use certain function of the image forming apparatus is logging in, function extension is executed for allowing the user to use the certain function temporary.