To produce a copolymer comprising a specific unit, having a high viscosity of an aqueous solution and capable of manifesting excellent flocculating performances as a cationic polymeric flocculant.

This copolymer comprises substances represented y formulae I and II (R1 to R3 are each H, an aliphatic group or the like; R4 is H or methyl; R5 is a 2-5C alkylene). The copolymer has 10-10,000 cP Brookfield viscosity of a 1 wt.% aqueous solution at 25°C. The copolymer is preferably obtained by reacting a copolymer comprising substances represented by formulae II and III (R6 is H or methyl) [e.g. a copolymer of acrylonitrile and N- vinylpyrrolidone] with a polyamine (preferably a 1,2-diamine) in the liquid phase in the presence of ammonium chloride and/or amine hydrochloride, preferably at 60-200°C. The polyamine is preferably a compound represented by the formula H2NCHR7CHR8NHR9 (R7 to R9 are each H, an aliphatic group or the like).