The present invention provides a method for producing an optically active 4-[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoic acid or a pharmaceutically acceptable salt thereof, which includes subjecting a compound of the formula (I) wherein each symbol in the formula is as defined in the specification, to optical resolution by fractional crystallization to give an optically active compound thereof and subjecting the compound to hydrolysis reaction. According to this method, a resolution method useful for industrial large-scale production of the optically active compound of 4-[.alpha.-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoic acid, which is useful as a thromboxane synthetase inhibitor and an agent for the prophylaxis and treatment of diabetic complications, can be obtained.