Provided is a T cell activation inhibitor containing an RGM inhibiting substance such as an anti-RGM neutralizing antibody and the like as an active ingredient. The T cell activation inhibitor is useful as a pharmaceutical composition for the prophylaxis or treatment of autoimmune diseases such as multiple sclerosis and the like, and other diseases caused by T cell activation. In addition, a T cell activation inhibiting substance can be screened for by contacting a test substance with RGM and selecting a test substance that lowers the activity level of RGM.