The present invention relates to a function inhibitor of an effector cell comprising a CCR5 antagonist. Since the function inhibitor of an effector cell comprising a CCR5 antagonist inhibits the function of effector cells which play an important role in formation of diseases and the like, it is effective for the prevention and/or treatment of, for example, a transplant rejection (e.g., rejection of a solid organ graft, rejection of islet cell transplantation in diabetes mellitus, graft-versus-host disease (GVHD), etc.), an autoimmune disease (e.g., arthritis, rheumatoid arthritis, multiple sclerosis, ulcerative colitis, etc.), an allergic disease (e.g., asthma, etc.), an ischemic disease (e.g., ischemia-reperfusion injury, etc.), cancer or a metastatic disease or the like.