By using ion exchange column chromatography or calcium carbonate to an aqueous solution containing a compound A represented by the following formula and citric acid, citric acid in the aqueous solution is removed and a crystalline compound A is obtained by being thereafter subjected to several steps. Further a high-purity noncrystalline compound A is obtained by using calcium carbonate, sulfuric acid, an organic solvent and the like to the aqueous solution containing the compound A and citric acid to remove citric acid and the crystalline compound A in the solution. The configuration of the crystalline compound A is an SS isomer in the RS notation system. The configuration of the noncrystalline compound A is an SR isomer.
