A modified transposon vector and a method for introducing a foreign gene into a cell are provided. A modified transposon vector wherein a nucleic acid fragment having the following features (a)-(b) or (a)-(c) is inserted a cell expressing a foreign gene obtained by introducing said vector into chromosomal DNA of a cell line or an ontogenetic cell and further introducing an expression cassette of a foreign gene into the chromosomal DNA a genetically recombined animal generated by using said cell (in case of an ontogenetic cell) and a method for producing a foreign protein from said cell and said genetically recombined animal: (a) It consists of 5- and 3-TIR sequences of a transposon gene (b) A sequence where a recombination reaction occurs is inserted between at least either of DR regions present by twos each in 5- and 3-TIR sequences of a transposon gene (c) A restriction enzyme recognition site or an expression cassette of a foreign gene is inserted between 5- and 3-TIR sequences of a transposon gene.
