To provide a method that can discriminate whether or not image data come from an illegal copy and identify a source of the illegal copy.

A printer drier 25 has a data update function 32 that updates part of pixel data of image data 6 expressing 1st information 1 according to a prescribed algorithm in a dot pitch hardly visually recognized on the basis of 2nd information 2. When identification information 6a of a printer 6 is adopted for the 2nd information 2, the printer can print out the identification information 6a when the identification information 6a of the printer 6 is adopted as the 2nd information 2 without deteriorating the image quality of an output 8a of the 1st information 1. Thus, reading a printed matter 8 with an image reader such as a scanner can acquire the identification information 6a so as to identify the printer 6 used for printing out the printed matter 8.