a specially defined amount of hydrogen peroxide is used as a polymerization catalyzer ; a specially defined amount and a specially defined kind of a polyvalent metal ion is used ; and a specially defined amount of an alkaline substance is used , in order to polymerize a monomer component consisting essentially of only maleic acid , or a monomer component composed of maleic acid and another water - soluble ethylenically unsaturated monomer , in a specially defined proportion in water , whereby a maleic acid polymer salt , preferably a copolymer salt , of improved biodegradability is produced . the maleic acid polymer salt obtained is superior in biodegradability , has a low residual monomer content , and is useful as a detergent builder and as a scale inhibitor , etc .