This invention relates to a method for producing a protein of interest, comprising introducing a protein expression vector which comprises a gene fragment a gene fragment comprising a DNA encoding a protein of interest and a selectable marker gene and transposon sequences at both terminals of the gene fragment, into a suspension mammalian cell; integrating the gene fragment inserted between a pair of the transposon sequences, into a chromosome of the mammalian cell to obtain a mammalian cell capable of expressing the protein of interest; and suspension-culturing the mammalian cell; and a suspension mammalian cell capable of expressing the protein of interest.
