A process for producing an aqueous dispersion of a water-soluble cationic polymer which process comprises modifying a polymer obtained by homopolymerization of an N-vinylcarboxylic acid amide or copolymerization of an N-vinylcarboxylic acid amide and a vinyl monomer copolymerizable with the N-vinyl carboxylic acid amide with an acid in an aqueous medium in the presence of a nitrate ion. The process can produce an aqueous dispersion of a water-soluble cationic polymer having a high molecular weight from the above water-insoluble polymer easily in a single step without causing adverse effects on the handling property while the fluidity is maintained.