The purpose of the invention is to provide means with which it is possible to efficiently select a vector to which a foreign gene has been introduced when a foreign gene is to be introduced by homologous recombination to a vector having multiple sequences homologous with one another. The vector comprises, in succession, a replication origin, a sequence A, a marker gene X, two sequences C and D for introducing a foreign gene by homologous recombination, and a sequence B homologous with sequence A. The two sequences C and D are directly or indirectly adjacent to one another. The vector is used for introducing a foreign gene between the two adjacent sequences C and D.