In the optical resolution of an imidazole compound by formation of salt with L-10-camphorsulfonic acid, an efficient resolution method, namely the preferential dissolution method, was established. After precipitation of both isomers＇ salts completely from a minimum volume of ethanol aqueous solution, an ethanol aqueous solution is added to the Crystallization Solution stepwise to dissolve preferentially the unwanted isomer. Optical purity and yield do not depend on the addition method, but on the Volume of solvent and are estimated with good agreement from their solubilities. In comparison with the conventional cooling method, (+)imidazole compound is steadily obtained with higher yield (50%) and higher optical purity (97%). On the basis of these results, the method is applied to a 1700 l-crystallizer using a power number per volume as a scale up factor and good reproducibility is conformed.