PURPOSE: Acrylic acid is neutralized with an alkali or ammonium in an amount less than the equevalent and polymerized at a specific temperature, heat treated, thus producing title crosslinking high polymer that absorbs and expells water in high level and can be used for food and medcal materials.

CONSTITUTION: Acrylic acid is neutralized with an alkali or ammonium or their combination in an amount less than the equivalent, polymerized at a temperature lower than 100°C and heat treated at a temperature higher than 106°C. The heat treatment causes the crosslinking by the unneutralized carboxyl groups to procuce said insoluble and swelling high polymr. The resulting swelling high polymer reversibly absorbs and expells water in high level and requires no crosslinking agent, thereby it can be employed in various kinds of application as food, medical and hygienic application or plant cultivation.