PURPOSE: To produce the titled compound useful as a radical generator, in high efficiency, without producing by-products, by reacting an aqueous solution of an alkali metal salt of hydroperoxide with an acid halide, separating an organic layer from the reaction product, and cooling the layer at a specific temperature.

CONSTITUTION: An aqueous solution of an alkali metal salt of the hydroperoxide of formula I (R1 is 1W6C straight or branched-chain alkyl, phenyl, etc.) is made to react with an acid halide of formula II (R2 is 1W20C strainght or branched- chain alkyl, phenyl, etc.; X is halogen), and an organic layer composed mainly of the peroxy ester of formula III is separated from the reaction product. The layer is optionally washed with an aqueous solution of an alkali or a neutral salt, and cooled at -1W-60°C to separate and remove the condensed water, by-products and unreacted raw materials dispersed in the organic layer and obtain the objective peroxy ester.