A compound represented by the formula: wherein R 1, R 2 and R 3 each independently represent a hydrogen atom, a halogen atom, a cyano group, a nitro group, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, an acyl group, an optionally substituted amino group, an optionally substituted carbamoyl group, an optionally substituted thiocarbamoyl group, an optionally substituted sulfamoyl group, an optionally substituted hydroxy group, an optionally substituted sulfanyl group or an optionally substituted silyl group; R 4 represents an optionally substituted aromatic heterocyclic group; R 5a and R 6a each independently represent a hydrogen atom, a halogen atom, a cyano group, a nitro group, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, an acyl group, an optionally substituted amino group, an optionally substituted carbamoyl group, an optionally substituted thiocarbamoyl group, an optionally substituted sulfamoyl group, an optionally substituted hydroxy group, an optionally substituted sulfanyl group or an optionally substituted silyl group; R 5b and R 6b together (i) form a double bond or (ii) form an optionally substituted C 3-4 cycloalkyl together including the carbon atom to which they are mutually bound, or each independently represent a hydrogen atom, a halogen atom, a cyano group, a nitro group, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, an acyl group, an optionally substituted amino group, an optionally substituted carbamoyl group, an optionally substituted thiocarbamoyl group, an optionally substituted sulfamoyl group, an optionally substituted hydroxy group, an optionally substituted sulfanyl group or an optionally substituted silyl group; and R 7 and R 8 each independently represent a hydrogen atom, a halogen atom, a cyano group, a nitro group, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, an acyl group, an optionally substituted amino group, an optionally substituted carbamoyl group, an optionally substituted thiocarbamoyl group, an optionally substituted sulfamoyl group, an optionally substituted hydroxy group, an optionally substituted sulfanyl group or an optionally substituted silyl group, or form an optionally substituted nitrogen-containing heterocycle together including the nitrogen atom to which they are mutually bound, or a salt thereof. R 1, R 2 and R 3 each independently represent a hydrogen atom, a halogen atom, a cyano group, a nitro group, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, an acyl group, an optionally substituted amino group, an optionally substituted carbamoyl group, an optionally substituted thiocarbamoyl group, an optionally substituted sulfamoyl group, an optionally substituted hydroxy group, an optionally substituted sulfanyl group or an optionally substituted silyl group; R 4 represents an optionally substituted aromatic heterocyclic group; R 5a and R 6a each independently represent a hydrogen atom, a halogen atom, a cyano group, a nitro group, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, an acyl group, an optionally substituted amino group, an optionally substituted carbamoyl group, an optionally substituted thiocarbamoyl group, an optionally substituted sulfamoyl group, an optionally substituted hydroxy group, an optionally substituted sulfanyl group or an optionally substituted silyl group; R 5b and R 6b together (i) form a double bond or (ii) form an optionally substituted C 3-4 cycloalkyl together including the carbon atom to which they are mutually bound, or each independently represent a hydrogen atom, a halogen atom, a cyano group, a nitro group, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, an acyl group, an optionally substituted amino group, an optionally substituted carbamoyl group, an optionally substituted thiocarbamoyl group, an optionally substituted sulfamoyl group, an optionally substituted hydroxy group, an optionally substituted sulfanyl group or an optionally substituted silyl group; and R 7 and R 8 each independently represent a hydrogen atom, a halogen atom, a cyano group, a nitro group, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, an acyl group, an optionally substituted amino group, an optionally substituted carbamoyl group, an optionally substituted thiocarbamoyl group, an optionally substituted sulfamoyl group, an optionally substituted hydroxy group, an optionally substituted sulfanyl group or an optionally substituted silyl group, or form an optionally substituted nitrogen-containing heterocycle together including the nitrogen atom to which they are mutually bound, or a salt thereof.