The present invention provides with a therapeutic agent for arthritis, which contains a low-molecular-weight FoxM1 inhibitor, e.g., thiostrepton, as an active ingredient. FoxM1 is expressed in an osteoclast progenitor that occurs specifically in an inflamed synovium in arthritis, and is not involved in the function of an osteoclast that is present in the bone marrow in a normal person and is involved in normal bone metabolism. Therefore, arthritis can be prevented and/or treated without any adverse side effect by using a FozMl inhibitor.