A pyrimidone derivative represented by formula (I) or a salt thereof, or a solvate thereof or a hydrate thereof having inhibitory activity against tau protein kinase 1: wherein R.sup.1 represents a C.sub.1-C.sub.12 alkyl group which may be substituted; R represents, for example, a group represented by the following formula (II): wherein R.sup.2 and R.sup.3 independently represent a hydrogen atom or a C.sub.1-C.sub.8 alkyl group; R.sup.4 represents a benzene ring which may be substituted, a naphthalene ring which may be substituted, an indan ring which may be substituted, a tetrahydronaphthalene ring which may be substituted, or an optionally substituted heterocyclic ring having 1 to 4 hetero atoms selected from the group consisting of oxygen atom, sulfur atom and nitrogen atom, and having 5 to 10 ring-constituting atoms in total. ##STR00001##