The present invention relates to a novel use of regulatory T cell-specific surface protein Lrig-1, and more specifically to an immunosuppressive agent comprising siRNA which inhibits the expression of surface protein Lrig-1. In addition, the invention relates to a method for screening an immunosuppressive agent which inhibits proteins of Lrig-1 or genes encoding the proteins. As a result, an immunosuppressive agent with low side effects and high specificity can be developed.