A process is provided for producing a vinyl chloride-based polymer by polymerization of vinyl chloride or a monomer mixture containing it in a reaction vessel. Heat generated during polymerization is removed using a reflux condenser. When the polymerization rate is within a range from 30% to 50%, a copolymeric polyether, with a weight average molecular weight of 1,500,000 to 2,000,000, and an ethylene oxide to propylene oxide molar ratio within a range from 78/22 to 82/18, is added to the reaction mass. Foaming of the polymer slurry due to the use of the reflux condenser beyond the point where the polymerization rate reaches 60% is suppressed, and the polymer can be produced with no deleterious effects on the quality of the product polymer.