1. An image processor comprising: a memory; and a microprocessor-based control system configured to execute an image processing program stored in the memory to cause the image processor to: accept an input of user authentication information comprising a first type of login information to login a first user to the image processor, perform user authentication by comparing the inputted user authentication information with stored user authentication information to authorize a first use of the image processor, in response to the user authentication being successful, authorize the first use of the image processor, automatically generate a temporary ID comprising a second type of login information, notify the first user of the generated temporary ID and store the generated temporary ID as related to the stored user authentication information, accept a logout to end the first use of the image processor, after the generation of the temporary ID and the accepting of the logout to end the first use of the image processor, accept an input of the temporary ID, instead of said user authentication information comprising the first type of login information, to login the first user to the image processor, compare the inputted temporary ID with the stored temporary ID to thereby perform the user authentication and to authorize a second use of the image processor when the inputted temporary ID and the stored temporary ID match with each other, and determine that a second temporary ID, matching the stored temporary ID and previously inputted by a second user to login to the image processor, is invalid when the first user succeeds in user authentication based on the user authentication information comprising the first type of login information instead of the temporary ID, wherein the determination that the inputted second temporary ID is invalid causes use of the image processor by the second user to be stopped and the second user to be forcibly logged out.