The present invention provides a process for preparing various polymorphic modifications of 2-(3-cyano-4-isobutyloxyphenyl)-4-methyl-5-thiazolecarboxylic acid each selectively by crystallization under the specific conditions determined by temperature and methanol/water ratio of the mixed solvent to be used and polymorphic modifications of 2-(3-cyano-4-isobutyloxyphenyl)-4-methyl-5-thiazolecarboxylic acid as obtained by this process. A process of subjecting a specific polymorphic modification of the above acid to heating and vacuum drying to thereby obtain another polymorphic modification of the acid or an amorphous modification thereof and polymorphic or amorphous modifications of the acid as obtained by this process.
