The present invention relates to a method for producing a phosphoric acid ester-based polymer, including copolymerizing a monomer 1 represented by the following formula (1), a monomer 2 represented by the following formula (2), and a monomer 3 represented by the following formula (3), at pH 7 or lower, in the present of a polymerization solvent having a dissolved oxygen concentration at 25.degree. C. of 0.01 to 4.0 mg/kg. The invention also relates to a method for producing a phosphoric acid ester-based polymer, including copolymerizing the monomers 1, 2 and 3 at pH 7 or lower, in the presence of a phosphonic acid-based chelating agent. The invention also relates to a method for producing a phosphoric acid ester-based polymer from a mixed solution containing the monomer 1, 2 and 3, including obtaining a mixed solution by mixing the monomers 1 to 3 at a temperature of 10 to 50.degree. C., initiating a polymerization within 72 hours after the mixing of the monomers 1 to 3, and maintaining the temperature of the mixed solution in the range of 10 to 50.degree. C. until the polymerization is initiated: ##STR00001## wherein R.sup.1 and R.sup.2 each represent a hydrogen atom or a methyl group; and R.sup.3 represents a hydrogen atom or --COO(AO).sub.nX [wherein AO represents an oxyalkylene group or oxystyrene group having 2 to 4 carbon atoms; n represents an average number of added moles of AO, and is a number from 3 to 200; and X represents a hydrogen atom or an alkyl group having 1 to 18 carbon atoms], ##STR00002## wherein R.sup.4 represents a hydrogen atom or a methyl group; R.sup.5 represents an alkylene group having 2 to 12 carbon atoms; m1 represents a number from 1 to 30; and M represents a hydrogen atom, an alkali metal or an alkaline earth metal, and ##STR00003## wherein R.sup.6 and R.sup.8 each represent a hydrogen atom or a methyl group; R.sup.7 and R.sup.9 each represent an alkylene group having 2 to 12 carbon atoms; m2 and m3 each represent a number from 1 to 30; and M represents a hydrogen atom, an alkali metal or an alkaline earth metal.