PROBLEM TO BE SOLVED: To provide a novel poly(ADP-ribose) polymerase activity inhibitor.SOLUTION: The present invention provides a 8,9-dihydro-2H-pyrido[4,3,2-de]phthalazin-3(7H)-one compound represented by formula (I) and the like.SELECTED DRAWING: None【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