A compound that can be a therapeutic agent for underactive bladder which has low risk of side effects and can be administered orally is provided. Furthermore, a crystal which is advantageous in view of the stability, long-term storage, handleability, easiness of the drug preparation or the like as a drug substance of medicaments is provided. Since a salt 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 diastereomeric mixture thereof and 4-piperidinemethanol is a chemically very stable crystal, it can be stored for a long time, and is very useful as a drug substance of medicaments.