Provided is a novel bicyclic compound which is capable of inhibiting HSP90 and has a carcinostatic effect. A compound represented by general formula (I) or a salt thereof. In general formula (I): at least one of X1, X2, X3 and X4 represents N or N-oxide and the remainders are the same or different and represent C-R2 one or two of Y1, Y2, Y3 and Y4 represent C-R4 and the remainders are the same or different and represent CH or N R1 represents an optionally substituted monocyclic or bicyclic unsaturated heterocyclic group having 1-4 hetero atoms, said hetero atoms being selected from among N, S and O R2 represents a hydrogen atom, an optionally substituted C1-6 alkyl group, etc. R3 represents -CO-R5, etc. R4 represents a hydrogen atom, -N(R7)(R8), etc. R5 represents a hydroxyl group, an amino group, etc. and R7 and R8 are the same or different and represent a hydrogen atom, an optionally substituted C1-6 alkyl group R9 represents an optionally substituted C3-7 cyclic alkyl group, etc.本發明提供一種具有HSP90抑制作用,且具有癌細胞增殖抑制效果的新穎之雙環式化合物,另外,本發明提供一種基於HSP90抑制作用,而可用於預防及/或治療該HSP90參與之疾病、尤其是癌的醫藥。本發明係下述通式(I)所表示之化合物或其鹽:(式中,X1、X2、X3及X4中至少1個為N或N氧化物,其餘相同或不同地表示C-R2;Y1、Y2、Y3及Y4中任意1個或2個為C-R4,其餘相同或不同地表示CH或N;R1表示可具有取代基的具有選自N、S及O中之1~4個雜原子的單環性或雙環性之不飽和雜環基;R2表示氫原子、可具有取代基之碳數1~6之烷基等;R3表示氫原子、-CO-R5等;R4表示氫原子、-CO-R6、-N(R7)(R8)等;R5表示羥基、胺基等;R6表示羥基等;R7及R8相同或不同地表示氫原子、可具有取代基之碳數1~6之烷基等;R9表示可具有取代基之碳數3~7之環烷基等)
