The present invention makes it possible to efficiently produce an N-methyl transferase with 7-methylxanthine N3 methyl transferase, theobromine N1 methyl transferase, and paraxanthine N3 methyl transferase activities that can be utilized as an industrial, food, or medical enzyme. In addition, the present invention makes it possible to modify the caffeine biosynthesis metabolism of caffeine producing plants, plant tissues, or plant cells, thereby efficiently producing caffeine metabolism based compounds or modifying the production of caffeine in the plants.