An antibody against AILIM (alternatively called JTT-1 antigen, JTT-2 antigen, ICOS and 8F4) was found to have a significant therapeutic effect on arthrosis, for example, rheumatoid arthritis and osteoarthritis, graft versus host disease, graft immune rejection, inflammation (hepatitis and inflammatory bowel diseases), diseased condition accompanied by the excessive production of an antibody against a foreign antigen triggered by immunological sensitization by the antigen.