Method for producing phosphoric acid ester-based polymer by copolymerizing monomer 1 of formula (1), monomer 2 of formula (2), and monomer 3 of formula (3): wherein the variables are as defined in the specification; wherein the variables are as defined in the specification; and wherein the variables are as defined in the specification, at pH ≦7 in solvent having dissolved oxygen concentration of 0.01-4.0 mg/kg, or in presence of phosphonic acid-based chelating agent, or by mixing monomers 1 to 3 at 10-50 ° C., initiating polymerization within 72 hours, and maintaining the temperature at 10-50 ° C. until polymerization is initiated. The phosphoric acid ester-based polymer may be used as a dispersant in a hydraulic composition.