The present invention relates to a method for effectively proliferating regulatory T cells, by which, particularly, in the presence of a fusion protein dimer comprising IL-2 protein or a variant thereof and CD80 protein or a fragment thereof, CD4+, CD25+, and CD127− T cells can be effectively proliferated. In particular, when combined with a predetermined cell culture medium, regulatory T cells such as CD4+, CD25+, and CD127− can be effectively and specifically proliferated. In addition, when the method is used, it has been confirmed that the survival rate of regulatory T cells is remarkably increased as compared to a conventionally used culture method using IL-2, and a significant increase in the yield of Foxp3+ regulatory T cells has been confirmed. Thus, such a proliferation method can be used in the field of cell therapeutic agents using regulatory T cells.