A method of efficiently extracting a high-purity stereoisomer from a mixture comprising the endo isomer and the exo isomer of a dicarboxylic acid having a norbornene or norbornane structure, or a derivative thereof. The present invention relates to a method of separating the endo isomer and the exo isomer of a dicarboxylic acid represented by a general formula (1) or (2) or a derivative thereof, and includes the step of stirring and mixing a mixture comprising mainly the endo isomer of the dicarboxylic acid represented by the general formula (1) or (2) or a derivative thereof, and the exo isomer of the dicarboxylic acid represented by the general formula (1) or (2) or a derivative thereof, with a basic compound and a solvent. ##STR00001## (wherein, R.sub.1 to R.sub.8 represent a hydrogen atom, methyl group, ethyl group, or butyl group), ##STR00002## (wherein, R'.sub.1 to R'.sub.10 represent a hydrogen atom, methyl group, ethyl group, or butyl group).