The invention provides a therapeutic agent for autoimmune diseases, particularly type 1 diabetes and rheumatoid arthritis, as well as a method of preparing the therapeutic agent and a method of treating an autoimmune disease with the therapeutic agent. The therapeutic agent is an anti-CD98 antibody that does not inhibit amino acid transport of CD98 but has a suppressive action on T lymphocyte activation, a suppressive action on cell adhesion mediated by fibronectin, as well as an OVA antibody production suppressive action in OVA immunization model.
