PURPOSE: To obtain the subject compd. high in water absorbability, harmless to a living body and used for a medicine carrier by adding alginate to a polyvalent gel forming cation-containing aq. salt soln. and substituting the aq. soln. with an org. solvent to react the same with a crosslinking agent and substituting the reaction product with a monovalent cation.

CONSTITUTION: An alginate aq. soln. is added to an aq. salt soln. containing at least one polyvalent gel forming cation and this aq. soln. is substituted with a mixture of an org. solvent capable of dissolving a crosslinking agent or solvents to react the alginate gel suspended in the org. solvent with the crosslinking agent and the reaction product is washed to remove the crosslinking agent and the gel forming cation of an aq. phase is substituted by a monovalent cation forming no gel to dry the aq. phase to obtain a crosslinked alkali metal or ammonium alginate gel swollen in its vol. by 10-300 times by water or 2-70 times by a physiological saline soln. and keeping the first shape even after swelling.