A pyrimidone derivative represented by formula (I) or a salt thereof: Wherein: R1 represents a hydrogen atom or a C1-6 alkyl group which may be substituted by a C6,10 aryl group; R2 represents a C1-10 alkyl group which may be substituted, a C2-6 alkenyl group which may be substituted, a C3-6 alkynyl group which may be substituted, a C3-6 cycloalkyl group which may be substituted, or a C6-10 ARYL group which may be substituted; or R1 and R2 form together a C2-6 alkylene group which may be substituted; or R1 and R2 form together a chain of formula —(CH2)2—X—(CH2)2— or —(CH2)2—X—(CH2)3— where X represents a oxygen atom, a sulfur atom, or a nitrogen atom which may be substituted; R3 represents a 2, 3 or 4-pyridyl group optionally substituted by a C1-4 alkyl group, C1-4 alkoxy group or halogen atom; and R4 represents a C1-10 alkyl group optionally substituted by a hydroxyl group, amino, C1-6 monoalkylamino group, C2-12 dialkylamino group or C6,10 aryl group which may be substituted. And a medicament comprising the said derivative or a salt thereof as an active ingredient which is used for preventive and/or therapeutic treatment of a neurodegenerative disease caused by abnormal activity of GSK3β such as Alzheimer's disease.