PURPOSE: To isolate the titled compound useful as a biochemical reagent, etc., ecnomically in an industrial scale, from an aqueous solution of specific lactone mixture, by maintaining the pH of the solution within a range to open the lactone ring of the titled compound exclusively, separating the formed acid from the other lactones, and then closing the ring to regenerate the objective lactone.

CONSTITUTION: An aqueous solution of a mixture of (A) 1,2-O-isopropylidene- β-L-idurono-6,3-lactone and (B) 1,2-O-isopropylidene-α-D-glucurono-6,3-lactone is added with a base such as sodium hydroxide while maintaining the pH of the solution within the range under which only the lactone ring of the component A opens, i.e. at pH6W7. The products are subjected to the thin layer chromatography, etc. to separate the produced induronic acid from the component B. The separated iduronic acid is dissolved in water, passed through a column filled with a cation exchange resin, and the effluent is concentrated under reduced pressure to obtain the objective compound.