A technique for efficiently introducing a foreign gene into cells with the use of transposons. In particular, a technique for efficiently preparing a transgenic organism with the use of a transposon having its transposition activity strikingly enhanced through methylation of a sequence containing the transposon. The methylation is retained even after incorporation in a genome, and now can be utilized in actual gene incorporation in a genome. This technique can realize strikingly efficient gene transformation as compared with the a method of preparing a transgenic organism with the use of conventional transposons.