The invention provides a process wherein a dispersant for a hydraulic composition capable of conferring an excellent dispersing effect and a viscosity reducing effect on a hydraulic composition can be produced at an industrially practical level. A phosphate polymer preferable as a dispersant for a hydraulic composition is obtained by copolymerization of a specific monomer 1 having a polyoxyalkylene group, a phosphoric monoester monomer 2 and a phosphoric diester monomer 3 at pH 7 or less.