A pyrimidone derivative represented by formula or a salt thereof, or a solvate thereof or a hydrate thereof: wherein: Y represents two hydrogen atoms, a sulphur atom, an oxygen atom or a C 1-2 alkyl group and a hydrogen atom; Z represents a bond, an oxygen atom, a nitrogen atom substituted by a hydrogen atom or a C 1-3 alkyl group, a sulphur atom, a methylene group optionally substituted by one or two groups chosen from a C 1-6 alkyl group, a hydroxyl group, a C 1-6 alkoxy group, a C 1-2 perhalogenated alkyl group or an amino group; R1 represents a 2, 3 or 4-pyridine ring or a 2, 4 or 5-pyrimidine ring, the ring being optionally substituted by a C 1-6 alkyl group, a C 1-6 alkoxy group or a halogen atom; R2 represents a hydrogen atom, a C 1-6 alkyl group or a halogen atom; R3 represents a benzene ring or a naphthalene ring; the rings being optionally substituted by 1 to 4 substituents selected from a C 1-6 alkyl group, a halogen atom, a C 1-2 perhalogenated alkyl group, a C 1-3 halogenated alkyl group, a hydroxyl group, a C 1-6 alkoxy group, a C 1-2 perhalogenated alkoxy group, a C 1-6 alkylsulfonyl group, a nitro, a cyano, an amino, a C 1-6 monoalkylamino group, a C 2-12 dialkylamino group, an acetoxy group or an aminosulfonyl group, a 4-15 membered heterocyclic group, this group being optionally substituted by a C 1-6 alkyl group, a halogen atom, a C 1-2 perhalogenated alkyl group, a C 1-3 halogenated alkyl group, a hydroxyl group, a C 1-6 alkoxy group; R4 represents a hydrogen atom; a C 1-6 alkyl group; a benzyl group, a phenethyl group, a benzyloxycarbonyl group, a C 1-4 alkoxy carbonyl group, a benzene group, a naphthalene group, a 5,6,7,8-tetrahydronaphthalene group, a benzenesulfonyl group, a benzoyl group, a 4-15 membered heterocyclic group, the above-mentioned group being optionally substituted by 1 to 4 substituents selected from a halogen atom, a hydroxyl group, a C 1-4 alkoxy group; a C 1-2 perhalogenated alkyl group, a C 1-6 alkyl group, a benzene group, a halogen atom,,a C 1-3 halogenated alkyl group, a nitro, a cyano, an amino, a C 1-6 monoalkylamino group or a C 2-10 dialkylamino group; R5 represents a hydrogen atom or a C 1-6 alkyl group; and n represents 0 to 3. Y represents two hydrogen atoms, a sulphur atom, an oxygen atom or a C 1-2 alkyl group and a hydrogen atom; Z represents a bond, an oxygen atom, a nitrogen atom substituted by a hydrogen atom or a C 1-3 alkyl group, a sulphur atom, a methylene group optionally substituted by one or two groups chosen from a C 1-6 alkyl group, a hydroxyl group, a C 1-6 alkoxy group, a C 1-2 perhalogenated alkyl group or an amino group; R1 represents a 2, 3 or 4-pyridine ring or a 2, 4 or 5-pyrimidine ring, the ring being optionally substituted by a C 1-6 alkyl group, a C 1-6 alkoxy group or a halogen atom; R2 represents a hydrogen atom, a C 1-6 alkyl group or a halogen atom; R3 represents a benzene ring or a naphthalene ring; the rings being optionally substituted by 1 to 4 substituents selected from a C 1-6 alkyl group, a halogen atom, a C 1-2 perhalogenated alkyl group, a C 1-3 halogenated alkyl group, a hydroxyl group, a C 1-6 alkoxy group, a C 1-2 perhalogenated alkoxy group, a C 1-6 alkylsulfonyl group, a nitro, a cyano, an amino, a C 1-6 monoalkylamino group, a C 2-12 dialkylamino group, an acetoxy group or an aminosulfonyl group, a 4-15 membered heterocyclic group, this group being optionally substituted by a C 1-6 alkyl group, a halogen atom, a C 1-2 perhalogenated alkyl group, a C 1-3 halogenated alkyl group, a hydroxyl group, a C 1-6 alkoxy group; R4 represents a hydrogen atom; a C 1-6 alkyl group; a benzyl group, a phenethyl group, a benzyloxycarbonyl group, a C 1-4 alkoxy carbonyl group, a benzene group, a naphthalene group, a 5,6,7,8-tetrahydronaphthalene group, a benzenesulfonyl group, a benzoyl group, a 4-15 membered heterocyclic group, the above-mentioned group being optionally substituted by 1 to 4 substituents selected from a halogen atom, a hydroxyl group, a C 1-4 alkoxy group; a C 1-2 perhalogenated alkyl group, a C 1-6 alkyl group, a benzene group, a halogen atom,,a C 1-3 halogenated alkyl group, a nitro, a cyano, an amino, a C 1-6 monoalkylamino group or a C 2-10 dialkylamino group; R5 represents a hydrogen atom or a C 1-6 alkyl group; and n represents 0 to 3.