PURPOSE: To obtain the titled copolymer useful as a flocculating agent of high polymer, having a cationic group stably, by copolymerizing a specific amino group-containing monomer with a vinyl ester, saponifying the copolymer, quaternizing it with an acid or an alkylating quaternizing agnet.

CONSTITUTION: A monomer [e.g., N-(3-allyloxy-2-hydroxypropyl)dimethylamine acetate, etc.) shown by the formula I (A is group shown by the formula II, formula III, etc.; R1 is lower alkyl; X- is anion) is copolymerized with a vinyl ester (e.g., vinyl acetate, etc.) shown by the formula IV (R2 is H, or lower alkyl). The prepared copolymer is partially or completely saponified, the cationic copolymer is made into an aqueous solution, and reacted with an acid or an alkylating quaternizing agent, and quaternized, to give the desired copolymer.