3-Substituted-4-pyrimidone derivatives represented by formula (I) or salts thereof, solvates thereof or hydrates thereof, wherein R<1> represents a C1-C18 alkyl group which may be substituted or a C6-C14 aryl group which may be substituted; R<2> represents a C1-C18 alkyl group which may be substituted or a C7-C20 aralkyl group which may be substituted; 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 neurodegenerative diseases (e.g. Alzheimer disease) and the like.
