The invention relates to an amphoteric water-soluble polymer dispersion containing polymer particles with particle diameters of 100 .mu.m or smaller and produced by dispersion-copolymerization of a monomer mixture comprising a methacrylate monomer containing a quaternary ammonium salt group, an acrylate monomer containing a quaternary ammonium salt group, an anionic monomer, and a (meth)acrylamide in respectively ratios specified in an aqueous salt solution in the presence of a polymer dispersant soluble in the aqueous salt solution and the invention also relates to a dewatering agent for organic sludge, a pretreatment agent or yield improver for paper making raw materials, and/or a drainage properties-enhancing agent using the amphoteric water-soluble polymer.