A novel bicyclic compound having an HSP90 inhibitory effect and a carcinostatic effect. A pharmaceutical agent based on the HSP90 inhibitory effect and useful in the prevention and/or treatment of a disease involving HSP90, particularly cancer. The compound has a general formula (I) or is a salt thereof wherein at least one of X1, X2, X3, and X4 represents N or N-oxide and the rest thereof are each independently C—R2 any one or two of Y1, Y2, Y3, and Y4 represent C—R4 and the rest thereof are each independently CH or N R1 represents an optionally substituted monocyclic or bicyclic unsaturated heterocyclic group having 1 to 4 heteroatoms selected from N, S, and O R2 represents a hydrogen atom, an optionally substituted alkyl group having 1 to 6 carbon atoms etc. R3 represents a hydrogen atom, —CO—R5 etc.
