Disclosed is a process for producing A-form crystals of 2-(3-cyamo-4-isobutytoxyphenyl)-4-methyl-5-thiazotecarboxylic acid, comprising: a step of dissolving by heating 2-(3-cyano-4-isobutyloxyphenyl)-4-methyl-5-thiazolecarboxylic acid in one or a plurality of solvents as a good solvent, selected from the group consisting of 1-propmol, 2-propanol, ethanol, and acetone, a step of cooling the solution； and a step of adding to the solution a hydrocarbon solvent as a poor solvent.本發明提供一種2-(3-氰基-4-異丁氧基苯基)-4-甲基-5-噻唑羧酸之A結晶之製造方法，包含使2-(3-氰基-4-異丁氧基苯基)-4-甲基-5-噻唑羧酸以選自由1-丙醇、2-丙醇、乙醇及丙酮所組成群組之一種或複數種之溶劑作為良溶劑並加熱溶解之步驟，使該溶液冷卻之步驟，及於該溶液中添加烴溶劑作為不良溶劑之步驟。
