1. An image processor comprising: an input unit accepting an input of user authentication information from a user; a user authentication unit performing user authentication by comparing the inputted user authentication information with the stored user authentication information to authorize execution of processing; and a temporary ID generation unit generating a temporary ID in a case where the user authentication is successful to store the temporary ID, being related to the user authentication information, wherein the input unit accepts an input of the temporary ID and the user authentication unit compares the inputted temporary ID with the stored temporary ID to thereby perform the user authentication and to authorize execution of the processing.