PURPOSE: To enable optical resolution of (±)-α-ethylbenzylamine and (±)-2- benzamidocyclohexanecarboxylic acid salt at a low cost, by preferentially crystallizing a pair of hardly soluble diastereomer salt from a supersaturated solution of the above compounds.

CONSTITUTION: A supersaturated solution of (±)-α-ethylbenzylamine (EBA) and (±)-cis-2-benzamidocyclohexanecarboxylic acid (cis acid) salt is added with a small amount of a seed crystal of a hardly soluble diastereomer salt of either one of (+)-EBA.(-)-cis acid salt or (-)-EBA.(+)-cis acid salt to effect the preferential crystallization of the same kind of diastereomer salt. The obtained diastereomer salt is separated into a base and an acid using an alkali and an acid to obtain (+)-EBA and (-)-cis acid or (-)EBA and (+)-cis acid useful as an optical resolution agent for racemic acid or racemic base in high resolution efficiency and high optical purity.