Provided is a novel bicyclic compound which has an HSP90 inhibitory effect and a carcinostatic effect. Also provided is a pharmaceutical agent which is based on the HSP90 inhibitory effect and is useful in the prevention and/or treatment of a disease involving HSP90, particularly, cancer. The present invention provides a compound represented by the following general formula (I) or a salt thereof wherein at least one of X1, X2, X3, and X4 represents N or N-oxide and the rest thereof are the same or different and each represent C—R2; any one or two of Y1, Y2, Y3, and Y4 represent C—R4 and the rest thereof are the same or different and each represent 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.; R4 represents a hydrogen atom, —CO—R6, —N(R7)(R8) etc.; R5 represents a hydroxyl group, an amino group etc.; R6 represents a hydroxyl group etc.; R7 and R8 are the same or different and each represent a hydrogen atom, an optionally substituted alkyl group having 1 to 6 carbon atoms etc.; and R9 represents an optionally substituted cycloalkyl group having 3 to 7 carbon atoms etc.