The invention relates to an imidazo[1,2-a]pyrimidone derivative represented by formula (1) or a salt thereof, wherein: X represents a bond, an ethenylene group, and ethenylene group, a methylene group optionally substituted by one or two groups selected from a C.sub.1-6 alkyl group, a hydroxy group and a C.sub.1-4 alkoxy group; a carbonyl group, an oxygen atom, a sulfur atom, a sulfonyl group, a sulfoxide group or a nitrogen atom being optionally substituted by a C.sub.1-6 alkyl group; R1 represents a 2,3 or 4-pyridyl group optionally substituted by a C.sub.1-4 alkyl group, C.sub.1-4 alkoxy group or a halogen atom; R2 represents a C.sub.1-6 alkyl group, a C.sub.1-2 perhalogenated alkyl group, a C.sub.1-3 halogenated alkyl group, a benzyl group, a phenyl ring, a naphthyl ring, 5,6,7,8-tetrahydronaphthyl ring, a pyridyl ring, an indole ring, a pyrrole ring, a thiophene ring, a furan ring or an imidazole ring, the benzyl group and the rings being optionally substituted by 1 to 4 substituents selected from a C.sub.1-6 alkyl group, a phenyl ring, a halogen atom, a C.sub.1-2 perhalogenated alkyl group, a C.sub.1-3 halogenated alkyl group, a hydroxyl group, a C.sub.1-4 alkoxy group, a nitro, a cyano, an amino, a C.sub.1-6 monoalkylamino group or a C.sub.2-10 dialkylamino group; and n represents 0 to 3. The invention relates also to a medicament comprising the said derivative or a salt thereof as an active ingredient which is used for preventive and/or therapeutic treatment of a neurodegenerative disease caused by abnormal activity of GSK3.beta., such as Alzheimer disease.