A method of transferring a foreign gene into cells, characterized by involving: the step of transferring into the cells with the use of an adenovirus vector, a first nucleic acid, which has a sequence provided with adeno-associated virus-origin ITRs in both sides of the target foreign gene to be transferred, and a second nucleic acid, which has an adeno-associated virus-origin rep gene and a promoter for expressing this gene and carries a stuffer sequence inserted thereinto sandwiched in two recombinase recognition sequences and located between the rep gene and the promoter; and the step of expressing the Rep protein under the action of recombinase in the cells obtained in the above step to thereby integrate the target foreign gene into the chromosomal DNA.