Provided is a novel bicyclic compound having an HSP90 inhibitory action and a cancer cell growth inhibitory effect. In the following general formula (I) (wherein, X1, X2, X3 and X4 are at least one of N or N oxide and the other is the same or different and represents C—R2； Y1, Y2, Y3 and Y4) Is any one or two is C—R4, and the other is the same or different and represents CH or N； R1 may have a substituent, 1 selected from N, S and O Represents a monocyclic or bicyclic unsaturated heterocyclic group having -4 heteroatoms； R2 represents a hydrogen atom, an optionally substituted alkyl group having 1 to 6 carbon atoms, or the like； R3 represents —CO—R5 and the like； R4 represents a hydrogen atom, —N (R7) (R8) and the like； R5 represents a hydroxyl group, an amino group and the like； R7 and R8 are the same or different. And a hydrogen atom, an optionally substituted alkyl group having 1 to 6 carbon atoms, etc.). Or a salt thereof.ＨＳＰ９０阻害作用を有し、且つ癌細胞増殖抑制効果を有する新規な二環式化合物を提供。下記一般式（Ｉ）（式中、Ｘ１、Ｘ２、Ｘ３及びＸ４は、少なくとも１つはＮ又はＮオキシドであり、他が同一又は相異なってＣ－Ｒ２を示し；Ｙ１、Ｙ２、Ｙ３及びＹ４は、いずれか１つ又は２つがＣ－Ｒ４であり、他が同一又は相異なって、ＣＨ又はＮを示し；Ｒ１は置換基を有していてもよい、Ｎ、Ｓ及びＯから選ばれる１～４個のヘテロ原子を有する単環性又は二環性の不飽和複素環基を示し；Ｒ２は水素原子、置換基を有していてもよい炭素数１～６のアルキル基等を示し；Ｒ３は、－ＣＯ－Ｒ５等を示し；Ｒ４は、水素原子、－Ｎ（Ｒ７）（Ｒ８）等を示し；Ｒ５は、ヒドロキシル基、アミノ基等を示し；Ｒ７及びＲ８は、同一又は相異なって、水素原子、置換基を有していてもよい炭素数１～６のアルキル基等を示す。）で表される化合物又はその塩。
