A pyrimidone derivative represented by general formula (I) or a pharmaceutically acceptable salt thereof: wherein X represents hydrogen atom and Y represents hydroxyl group, or X represents fluorine atom and Y represents hydrogen atom; R1 represents a C1-6 alkyl group; R2 represents a morpholin-4-yl group which may be substituted, or the like, which is used for preventive and/or therapeutic treatment of a disease caused by tau protein kinase 1 hyperactivity such as a neurodegenerative diseases (e.g. Alzheimer disease).