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, etc.
