A pyrimidone derivative represented by formula (I) or a salts thereof: wherein R1 represents an alkyl group, an alkenyl group, an alkynyl group, a cycloalkyl group, an aryl group and the like; R2 represents a hydrogen atom, hydroxyl group, an alkyl group, an alkenyl group and the like, R3 represents a pyridyl group, and a medicament comprising said derivative or a salt thereof as an active ingredient which is used for preventive and/or therapeutic treatment of a disease caused by tau protein kinase l hyperactivity such as Alzheimer disease.