The present invention provides a technique of selectively producing a desired polymorph of 2-(3-cyano-4-isobutyloxyphenyl)-4-methyl-5-thiazolecarboxylic acid.The present invention also provides a method of producing various polymorphs of 2-(3-cyano-4-isobutyloxyphenyl)-4-methyl-5-thiazolecarboxylic acid, which comprises crystallizing under the conditions defined by a specific temperature and a composition of a mixed solvent of methanol and water, and polymorphs obtained by the method.The present invention further provides a method of producing the other polymorphs or amorphous compounds by drying a specific polymorph under a reduced pressure with heating, and the other polymorphs or amorphous compounds obtained by the method.
