A process for preparing an aqueous solution of an ionic copolymer having a copolymer concentration of 20 % by weight or less and a viscosity of 20 ps or more at 25.degree. C. which comprises conducting copolymerization of an acrylamide compound(B) with a diallylamine compound(A) by continuously adding an aqueous solution of (B) having a concentration of 30% by weight or less to an aqueous solution of the diallylamine compound(A) having a concentration of 5% by weight or more and less than 10% by weight, and this process exhibit high conversion of diallylamine compounds, of which reactivity is low, and a copolymer having a high molecular weight can be obtained.