To obtain a water-soluble or water-swellable polymer by adding an N-vinylcarboxamide to an ammonium salt of an acrylamidoalkylsulfonic acid, adding a small amount of a crosslinking substance having at least two olefinic double bonds to the mixture, and polymerizing the entire mixture.

An ammonium salt (49.99-98.99 pts.wt.) of an acrylamidoalkylsulfonic acid, represented by formula III (wherein R3 is H or methyl; and Z is a 1-4C alkylene) is dissolved in an alcoholic solvent, 1-50 pts.wt. N-vinylcarboxamide represented by formula IV (wherein R1 and R2 are each H or methyl) is added to the solution, and 0.01-5 pts.wt. crosslinking monomer having at least two double bonds is added to the solution. The resultant mixture is polymerized in the presence of a free-radical-forming compound. Thus, it is possible to obtain a water-soluble or water-swellable polymer containing crosslinkages derived from the crosslinking monomer, repeating units represented by formula I, and repeating units represented by formula II in a random manner and in a ratio equal to that of the weights of the added monomers.