To provide a method for producing the subject compound by which the molecular weight of the subject compound useful as a high molecular coagulant can be easily controlled by changing the reaction temperature of the reaction of a nitrile group-containing polymer with a polyamine.

The molecular weight of (C) polyvinylimidazoline or a salt thereof is controlled by changing the temperature of the reaction of (A) a nitrile group-containing polymer with (B) a polyamine (preferably 1,2-diamine). The reaction temperature is within the range of 60-200°C, and the higher the temperature, the lower the molecular weight of the polyvinylimidazoline, and the lower the temperature, the higher the molecular weight of the polyvinylimidazoline. The component A is preferably a methacrylonitrile homopolymer or copolymer. A chloride, a carboxylic acid or a metal salt is desirable as the catalyst. The reaction is preferably performed in a liquid phase. Thereby, the objective product is obtained without generating hydrogen sulfide as a byproduct.