in the present invention is not limited and includes preferably aprotic organic solvents for example aromatic hydrocarbons e g benzene xylene toluene etc halogenated hydrocarbons e g dichloromethane chloroform carbon tetrachloride dichloroethane trichlene perchlene monochlorobenzene dichlorobenzene etc esters e g ethyl acetate ethyl formate etc ethers e g diethyl ether tetrahydrofuran etc aliphatic or alicyclic hydrocarbons e g hexane heptane octane petroleum ether cyclohexane etc and a mixture thereof although the amount of the solvent varies largely depending on for example the kind of the solvent the kind of the isomerization catalyst and the like it is usually used in an amount of 0 5 to 20 fold by weight of the starting z isomer or e z isomer the reaction is usually carried out at a temperature of from 0 u00b0 to 20 u00b0 c preferably from 30 u00b0 to 150 u00b0 c for 0 5 to 48 hours the isomerization of z isomer into e isomer proceeds by treating z isomer or e z isomer with an acid and an isomerization catalyst by which a salt of e isomer is produced after the reaction the reaction mixture is cooled to precipitate the salt of e isomer from the mixture in general the salt of e isomer precipitates spontaneously as a.