The present invention relates to a method of selecting high producer clones by using an expression vector, the expression vector comprising: (i) a gene expression cassette comprising a selectable marker gene to which polyA has been inoperably linked; and (ii) a gene expression cassette which encodes a recombinant protein of interest and to which polyA has been operably linked. According to the invention, high producer clones can be selected from cell populations at least 10 times fewer than in the existing methods of selecting cell lines. Particularly, high producer clones can be selected using a low concentration of MTX compared to a conventional stepwise gene amplification strategy which comprises carrying out multiple amplification steps while increasing the concentration of MTX. Accordingly, the development period of cell lines can be shortened and the labor and cost required for selection of high-productivity cell clones can be reduced, whereby more efficient production of proteins is possible even when general selectable marker genes other than MTX are used.