PURPOSE: To obtain the titled compound with a high trans-content useful as an intermediate for liquid crystals, by heating an alkali or alkaline earth metal salt of a mixture of the cis- with the trans-isomers of the titled compound in an aqueous medium at a specific temperature.

CONSTITUTION: An alkali or alkaline earth metal salt of a mixture of the cis- with the trans-isomers of a compound expressed by the formula [R is 1W15C alkyl or cycloalkyl which may be substituted by alkoxyl, COOH or substituent group (carboxylic acid ester or phenyl)] as a free acid is heated in an aqueous medium which may contain an organic solvent at 150W300°C, preferably 200W 250°C to give the aimed compound with a high trans-isomer ratio in high yield without causing formation of by-products nor corrosion of apparatuses, etc. Examples of the above-mentioned organic solvent to be used include alcohols, e.g. methanol, ketones, e.g. acetone, and ethers, e.g. dioxane.