To obtain an anionic water-soluble vinyl polymer which has a high molecular weight, is uniformly dissolved in a water solution of a salt particularly the one containing a large amount of a multi-valency metal and exhibits an excellent aggregating and thickening effect by subjecting 2 kinds of specified vinyl monomers to copolymerization.

This anionic water-soluble vinyl polymer is a copolymer consisting of (A) a vinyl monomer as expressed by the formula CH2=C(R)- COOZSO3X (R is a hydrogen atom or a methyl group; X is an alkali metal or an alkaline earth metal; Z is a 7 or less C hydrocarbon group) and (B) a vinyl monomer as expressed by the formula CH2=C(R)-CONHZSO3X (R is a hydrogen atom or a methyl group; X is an alkali metal or an alkaline earth metal; Z is a 7 or less C hydrocarbon group). Preferably the molar number of A is 10-90% of the total molar number of A and B. This anionic water-soluble vinyl polymer has usually an intrinsic viscosity of 4 or more measured at 30°C in 1N-NaNo3.