PURPOSE: To obtain the titled compound useful as an intermediate for medicines and agricultural chemicals in high yield, by reacting a 2-hydroxyimino-3-keto-acid ester with a dialkyl sulfate in the presence of a basic substance in acetonitrile as a solvent.

CONSTITUTION: A 2-hydroxyimino-3-keto-acid ester of formula I (R1 is alkyl, etc.; R2 is lower alkyl) is reacted with a dialkyl sulfate, e.g. dimethyl sulfate, in the presence of a basic substance, e.g. a carbonate, in acetonitrile as a solvent at 15W 50°C for 5minW1hr. After the reaction, the resultant reaction solution is made weakly acidic, and the organic layer is concentrated to give the titled compound of formula II (R3 is lower alkyl).

EFFECT: The reaction proceeds under mild conditions by using acetonitrile as the solvent.