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 X 1 , X 2 , X 3 , and X 4 represents N or N-oxide and the rest thereof are the same or different and each represent C-R 2 ； any one or two of Y 1 , Y 2 , Y 3 , and Y 4 represent C-R 4 and the rest thereof are the same or different and each 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 a hydrogen atom, an optionally substituted alkyl group having 1 to 6 carbon atoms etc.； R 3 represents a hydrogen atom, -CO-R 5 etc.； R 4 represents a hydrogen atom, -CO-R 6 , -N(R 7 )(R 8 ) etc.； R 5 represents a hydroxyl group, an amino group etc.； R 6 represents a hydroxyl group etc.； R 7 and R 8 are the same or different and each represent a hydrogen atom, an optionally substituted alkyl group having 1 to 6 carbon atoms etc.； and R 9 represents an optionally substituted cycloalkyl group having 3 to 7 carbon atoms etc.
