PROBLEM TO BE SOLVED: To provide novel inhibitors of poly(ADP-ribose)polymerase activity.SOLUTION: This invention provides 8,9-dihydro-2H-pyrido[4,3,2-de]phthalazine-3-(7H)-one compounds represented by formula (I) or the like.【課題】新規ポリ(ADPリボース)ポリメラーゼ活性阻害剤の提供。【解決手段】式(I)等で表される8,9-ジヒドロ-2H-ピリド[4,3,2-de]フタラジン-3(7H)-オン化合物。【選択図】なし