The invention relates to a pyrimidone derivative represented by formula (I) or a salt thereof: ##STR00001## Wherein X, Y, R1, R2, R3, R4, n, p, q, o and m are as described herein. 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.