To readily obtain the subject optically high-purity chrysanthemum- monocarboxylic acid, by using a hydrophobic organic solvent as a separating solvent.

A process for reacting (A) (±)-trans-chrysanthemum- monocarboxylic acid or (±)-cis/trans-mixed chrysanthemum-monocarboxylic acid with (B) an optically active naphthylamine and a crystallizing and separating process for crystallizing a salt of one optically active trans-chrysanthemum- monocarboxylic acid and the component B and separating the salt from the mother liquor are carried out by using (C) a hydrophobic organic solvent (preferably an aromatic hydrocarbon such as benzene or the like or an aliphatic hydrocarbon such as hexane or the like). Preferably a compound of the formula (R1 and R2 are each H, an alkyl, an aralkyl or an aryl; R3 is a 1-6C alkyl; *is an asymmetric carbon) such as α-(1-naphthyl)ethylamine or the like is used as the component B.