An authentication apparatus for communicatively connecting to an image processing apparatus, comprising: - an authentication determination circuit that is adapted to determine, whether or not an input user ID entered by a user matches a registered user ID, and an input password entered by the user matches a registered password; - a partial determination circuit that is adapted to determine, whether or not any one of a first set or a second set exists among sets of the input user IDs and the input passwords having been entered hitherto, when the entered input user ID does not match the registered user ID or the entered input password does not match the registered password, the first set being a set where only the input user ID matches the registered user ID, the second set being a set where only the input password matches the registered password; and - a combination determination circuit that is adapted to generate at least one new set, when the first set or the second set exists, by combining the input user ID of the existing first set and the input password of at least one other set other than the first set, or by combining the input password of the existing second set and the input user ID of at least one other set other than the second set, and wherein the combination determination circuit is adapted to determine, whether or not the input user ID matches the registered user ID and the input password matches the registered password, among the generated at least one new set, and wherein the combination determination circuit is adapted to notify the authentication determination circuit, in case the input user ID matches the registered user ID and the input password matches the registered password among the generated at least one new set, wherein the authentication determination circuit is adapted to execute a user authentication if, among the generated at least one new set, the input user ID matches the registered user ID, and the input password matches the registered password. - an authentication determination circuit that is adapted to determine, whether or not an input user ID entered by a user matches a registered user ID, and an input password entered by the user matches a registered password; - a partial determination circuit that is adapted to determine, whether or not any one of a first set or a second set exists among sets of the input user IDs and the input passwords having been entered hitherto, when the entered input user ID does not match the registered user ID or the entered input password does not match the registered password, the first set being a set where only the input user ID matches the registered user ID, the second set being a set where only the input password matches the registered password; and - a combination determination circuit that is adapted to generate at least one new set, when the first set or the second set exists, by combining the input user ID of the existing first set and the input password of at least one other set other than the first set, or by combining the input password of the existing second set and the input user ID of at least one other set other than the second set, and wherein the combination determination circuit is adapted to determine, whether or not the input user ID matches the registered user ID and the input password matches the registered password, among the generated at least one new set, and wherein the combination determination circuit is adapted to notify the authentication determination circuit, in case the input user ID matches the registered user ID and the input password matches the registered password among the generated at least one new set,