Since an image processing system that can authenticate the originality of digital data adopts an arrangement that does not save and manage digital data in a server, the server scale can be prevented from increasing, and the security can be improved. A step difference that produces an optical path length for blue light which is equal to or longer than the wavelength of the blue light and optical path lengths for red and infrared light which are shorter than the wavelengths of the red and infrared light is utilized so as to exert an inverse action on the blue light to those exerted on the red and infrared light. Also disclosed is a method for an application.