Provided is a compound that can serve as a drug for the treatment of underactive bladder, which can be administered orally and poses little risk of side effects. Also provided are crystals that are advantageous as the bulk drug of a pharmaceutical in terms of stability, long-term storage, ease of handling, ease of formulation, and the like. Salts of 2-[(2-{(1R,5R)-2-oxo-5-[(1E,4S)-7,8,8-trifluoro-4-hydroxy-4-methyl-1,7‑octadien-1-yl]cyclopentyl}ethyl)thio]-1,3-thiazole-4-carboxylic acid, or a mixture of diastereomers thereof, and 4-piperidinemethanol are extremely chemically stable crystals and therefore permit long-term storage and are very useful as the bulk drug of a pharmaceutical.
