PURPOSE: To enhance the strength, elasticity and porosity of a gel, by hardening a water-containing gel formed of a water-soluble polyvalent metal salt, water and water-soluble polysaccharides by immersing said gel in a thick polyvalent metal salt aqueous solution or contacting the same with said solution.

CONSTITUTION: A water-containing gel is prepared from a water-soluble polyvalent metal salt, water, water-soluble polysaccharides and a hydrophylic org. liquid added if necessary and subsequently immersed in or contacted with a thick polyvalent metal salt aqueous solution and hardened to prepare a high strength water-containing gel. As the water-soluble polyvalent metal salt, there are aluminum acetate, aluminum sulfate, ferrous chloride and ferric chloride etc. and, as water-soluble polysaccharides, there are carboxy methyl cellulose and sodium alginate etc. The hydrophylic org. liquid is used in order to enhance the uniformity of the formed gel and there are glycerine, 1,3-butane diol and ethylene glycol, etc.