The pattern (pattern characteristic quantity) extracted from the pattern image carried by original printing paper is stored on the original printing paper and the validity of the original printing paper is verified according to the pattern stored on code-added printing paper at the time of copying the printed contents of the code-added printing paper so as to determine if the code-added printing paper is truly original or not by means of the pattern carried by the printing paper itself. Therefore, it is possible to prevent unauthorized copying with ease without using any special paper and hence appropriately protect the printed contents.