The invention relates to use of a pyrimidone derivative represented by formula (I) or a salt thereof wherein: X, Y, R1, R2, R3, R4, R5, m, n and p are as defined herein. In particular, the invention relates 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. or GSK3.beta. and cdk5/p25, such as Alzheimer disease.