PURPOSE:To provide a new 5-trifluoromethyl-1,3-dioxin-4-one derivative useful as an intermediate for 5-trifluoromethyluracil derivative useful as an antitumor agent or an antiviral agent or an intermediate raw material for other medicines, agricultural chemicals, etc. CONSTITUTION:The 5-trifluoromethyl-1,3-dioxin-4-one derivative of formula I (R1 and R2 are each a 1-5C alkyl or R1 and R2 together form a 3-6C alkylene; R3 is H, 1-6C alkyl or aryl), e.g. 4-oxo-5-trifluoromethyl-4H-1,3-dioxin-2- spirocyclohexane. This compound of formula I is obtained by reacting a compound of formula II (X is a halogen) with a trifluoromethylcopper in an inert solvent. Furthermore, this compound of formula I is made to react with a urea derivative to afford a 5-trifluoromethyluracil derivative useful as an antitumor agent or an antiviral agent in a high yield.