The invention relates to an use of pyrimidone derivative represented by formula (I) or a salt thereof: ##STR00001## wherein m, n, p, X, Y, R1, R2, R3, R4 and R5 are as defined herein for treating a variety of disease states. More specifically, 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's disease.