The present invention provides a prophylactic / therapeutic agent for arthritis including rheumatoid arthritis, a diagnostic kit, and a means for searching for a novel substance having preventive and / or therapeutic activity for arthritis including rheumatoid arthritis, comprising a Tet 3 expression inhibitor. To do.本発明はTet 3の発現阻害物質を含有する、関節リウマチを含む関節炎の予防・治療剤、診断キット、並びに関節リウマチを含む関節炎の予防及び/又は治療活性を有する新規な物質の探索手段を提供する。