A pyrimidone derivative represented by formula (I) or a salt thereof, or a solvate thereof or a hydrate thereof useful as a tau protein kinase 1 inhibitor: wherein X represents CH or nitrogen atom; R.sub.1 represents a C?1#191-C?12#191 alkyl group which may be substituted; R.sub.2 represents a C?1#191-C?8#191 alkyl group which may be substituted, 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##