PURPOSE: To provide the subject flocculant for high flocculation activity and adapted to the treatment of waste water, etc., obtained by the ternary polymerization of a (meth)acrylic ester having a polyalkylene oxide group, an acrylamide monomer and a quaternary ammonium salt-containing acrylic acid deriv.

CONSTITUTION: A terpolymer represented by the formula (wherein R is H or methyl, R' is H or a 1-6C alkyl, D is O or NH, E, G, J and J' are each H or a 1-6C alkyl, L, L' and L" are each H, a 1-18C alkyl, hydroxyalkyl or aryl, Q is a 1-6C alkyl, T is an anion such as chloride or a 1-4C alkyl sulfate, m is 1-2, n is 1-5 and x, y and z are 0.01-0.99 and the sum of them is 1) by the ternary polymerization of an acrylic ester monomer (A) such as hydroxyethyl methacrylate or ethoxyethoxyethyl acrylate, (meth)acrylamide (B) and a quaternary ammonium salt-containing monomer (C) such as acryloyloxyethyltrimethylammonium chloride.