Bicyclic compound or salt thereof

A compound represented by 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 the group consisting of N, S, and O;
• R2 represents a hydrogen atom, an optionally substituted alkyl group having 1 to 6 carbon atoms, an optionally substituted cycloalkyl group having 3 to 7 carbon atoms, or an optionally substituted alkenyl group having 2 to 6 carbon atoms;
• R3 represents a cyano group, or —CO—R5;
• R4 represents a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, —CO—R6, —N(R7)(R8), or —S—R9;
• R5 represents a hydroxyl group, an amino group, or an optionally substituted alkylamino group having 1 to 6 carbon atoms;
• R6 represents a hydroxyl group, an amino group optionally having a hydroxyl group, or an optionally substituted alkylamino group having 1 to 6 carbon atoms;
• 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, an optionally substituted cycloalkyl group having 3 to 7 carbon atoms, an optionally substituted aromatic hydrocarbon group, an optionally substituted saturated heterocyclic group, or an optionally substituted unsaturated heterocyclic group, or R7 and R8 together form a saturated heterocyclic group together with the nitrogen atom bonded thereto;
• and
• R9 represents an optionally substituted cycloalkyl group having 3 to 7 carbon atoms, or an optionally substituted aromatic hydrocarbon group.