A pyrimidone derivative having tau protein kinase 1 inhibitory activity which is represented by formula (I) or a salt thereof, or a solvate thereof or a hydrate thereof; useful for prventive and/or therapeutic treatment of diseass such as neurodegenerative diseases (e.g. Alzheimer disease); wherein Q represents CH or nitrogen atom; R represents a C.sub.1-C.sub.12 alkyl group; the ring of Formula (I): represents piperazine ring or piperidine ring; each X independently represents a C.sub.1-C.sub.8 alkyl group, an optionally partially hydrogenated C.sub.6-C.sub.10 aryl ring, an indan ring or the like; m represents an integer of 1 to 3; each independently represents a halogen atom, a hydroxy group, a cyano group, a C.sub.1-C.sub.6 alkyl group or the like; n represents an integer of 0 to 8; when X and Y or two Y groups are attached on the same carbon atom, they may combine to each other to form a C.sub.2-C.sub.6 alkylene group. ##STR00001##