A pyrimidone derivative represented by the formula (I) or a salts thereof: ##STR00001## wherein R.sup.1 represents an alkyl group, an alkenyl group, an alkynyl group, a cycloalkyl group, an aryl group and the like; R.sup.2 represents a hydrogen atom, hydroxyl group, an alkyl group, an alkenyl group and the like, R.sup.3 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 1 hyperactivity such as Alzheimer disease.