PURPOSE: To prepare a polymer of high water absorption, excellent in amylase resistance, by the graft-polymerization of starch thermally treated with an aqueous solution of butanol and acrylonitrile, followed by saponification.

CONSTITUTION: Starch is mixed with an aqueous solution of butanol and heated with reflexing, to form a butanol amylose composite, slightly soluble in water, and then the butanol is distilled off. An acrylonitrile monomer is added to the residual solution and polyacrylonitrile-starch graft-copolymer is obtained by graft-copolymerization of said composite and the acrylonitrile in the atmosphere of an inertgas, in the presence of a catalyst. Then the copolymer is saponified to give a polymer of high water absorption.