Vinyl chloride monomer, or a monomeric mixture comprising vinyl chloride as a main component and other monomers co-polymerizable therewith in an aqueous reaction medium, is polymerized in a polymerization vessel. All of the surfaces of the inside walls of the vessel and the agitator blades, the baffle plates and the like with which the vessel is equipped, are coated prior to the polymerization with at least one of a specific class of nitrogen-containing aromatic heterocyclic compounds. Not only is the deposition of polymer scale surprisingly reduced, but also the polymer thus obtained has good physical and chemical properties.