A printing method directed to a plurality of pieces of slip paper which have been bound into a book and to the outer surface, of which ink containing an initiator and either one of coloring matter and a precursor of coloring matter is applied are placed. A printing plate has a through portion through which radiation penetrates and whose shape corresponds to a pattern to be printed. The printing plate is placed ont he stacked slip paper and X-rays are irradiated upon the printing plate fro the above direction so as to print the pattern of a character on each pieces of the slip paper at a desired are thereof.