It is intended to provide a novel azabicyclo compound which exhibits both HSP90 inhibitory activity and cell proliferation inhibitory effect. Specifically disclosed is a compound represented by the following general formula (I) or a salt thereof: wherein X 1 represents CH or N； any one of X 2 , X 3 and X 4 represents N, and the others represent CH； any one or two of Y 1 , Y 2 , Y 3 and Y 4 represent C-R 4 , and the others are the same or different and represent CH or N； R 1 represents an optionally substituted monocyclic or bicyclic unsaturated heterocyclic group having 1 to 4 heteroatoms selected from N, S and O； R 2 represents an alkyl group having 1 to 6 carbon atoms, or the like； and R 3 and R 4 represent -CO-R 5 or the like.本發明提供一種具有HSP90抑制作用，且具有細胞增殖抑制效果之新穎之氮雜雙環式化合物。本發明之化合物或其鹽係由下述通式(I)所表示：(式中，X1表示CH或N；X2、X3及X4之任一者為N，其他表示CH；Y1、Y2、Y3及Y4之任一者或兩者為C-R4，其他相同或不同，表示CH或N；R1表示可具有取代基之具有選自N、S及O中之1~4個雜原子的單環性或雙環性不飽和雜環基；R2表示碳數1~6之烷基等；R3及R4表示-CO-R5等)。
