A method for highly efficiently preparing high purity crystals of 2-amino-2-[2-[4-(3-benzyloxyphenylthio)-2-chloro-phenyl]-ethyl]-1,3-propanediol hydrochloride. The method involves dissolving 2-amino-2-[2-[4-(3-benzyloxyphenylthio)-2-chlorophenyl]ethyl]-1,3-propanediol in a mixed solvent comprising a solvent in which its hydrochloride is highly soluble and a solvent in which its hydrochloride is less soluble, to prepare a solution of 2-amino-2-[2-[4-(3-benzyloxyphenylthio)-2-chlorophenyl]ethyl]-1,3-propanediol; and then adding hydrochloric acid to the resulting solution with stirring, to crystallize the hydrochloride of 2-amino-2-[2-[4-(3-benzyloxyphenylthio)-2-chloro-phenyl]ethyl]-1,3-propanediol.