This invention relates to a method of setting anti-forgery information addition level while taking an apparatus individual difference into consideration. The method is employed by a data processing system including: an image processing apparatus 100 for processing an image; and a maintenance apparatus 120, connected with the image processing apparatus through a network 110, for setting a processing function of the image processing apparatus 100. The maintenance apparatus 120 determines an anti-forgery information addition level depending on an individual difference of apparatuses, such as an exhaustion level of the image processing apparatus 100. The addition level determined by the maintenance apparatus 120 is transmitted to the image processing apparatus 100. The image processing apparatus 100 adds anti-forgery information to an inputted image at the addition level transmitted by the maintenance apparatus 120.