By performing ion exchange column chromatography on an aqueous solution containing a compound A represented by formula (A) and citric acid or by treating the aqueous solution with calcium carbonate, the citric acid in the aqueous solution is removed, and a crystalline compound A is obtained by subsequently subjecting the aqueous solution to several steps. In addition, a high-purity, non-crystalline compound A is obtained by treating the aqueous solution containing the compound A and the citric acid with calcium carbonate, sulfuric acid, an organic solvent, etc. to remove the citric acid and the crystalline compound A in the solution. The three-dimensional structure of the crystalline compound A is an SS stereoisomer according to the R-S system. The three-dimensional structure of the non-crystalline compound A is an SR stereoisomer according to the R-S system.
