PURPOSE: To obtain a water-base cross-linked polyurethane which is excellent in storage stability at a high solid content while having the toughness inherent in a polyurethane by reacting a polyurethane prepolymer self-dispersible in water with an isocyanate-reactive compd.

CONSTITUTION: An aminated polymer which is formed from a compd. of the formula (wherein R1 is H or a 1-3C alkyl; and one of R2 to R4 is a direct bond or a 1-3 C alkylene and the rest are each a 1-3C alkylene) having two hydroxyl groups and one mercapto group and a free-radical-polymerizable monomer component contg. an aminated free-radical-polymerizable monomer and has a number average mo.wt. of 1,000-20,000 is reacted with a diisocyanate compd. to give a polyurethane prepolymer having isocyanate groups at both molecular ends and self-dispersible in water. The prepolymer is dispersed in water and reacted with a polyamine compd. or a polyol compd. to give a water-base cross- linked polyurethane.