PURPOSE: To readily separate and purify the subject compound in high optical purity and good yield by treating a mixture of the subject compound with an optically active halohydrin with a basic compound, selectively converting the subject compound into an epoxy derivative and then fractionating the resultant epoxy derivative by distillation.

CONSTITUTION: A mixture of an optically active haloester expressed by formula I (A is 2-6C saturated or unsaturated alkylene; X is halogen; R is 1-20C alkyl or alkenyl; X and the ester group are in the trans form) with an optically active halohydrin expressed by formula II is treated in the presence of a basic compound to selectively convert only the halohydrin into an epoxy derivative expressed by formula III, which is then fractionated from the resultant mixture thereof with the haloester by distillation. The haloester is subsequently collected from the residue to readily separate and purify the optically active substance expressed by formula I useful as a raw material for medicines and agricultural chemicals from the mixture with the optically active substance expressed by formula II in high optical purity and good yield on an industrial scale.