There is provided a process for producing a vinyl chloride-based polymer, in which a suspension polymerization of either vinyl chloride monomer, or a mixture of vinyl chloride monomer and another copolymerizable monomer, is conducted in a polymerization vessel fitted with a reflux condenser, the process including the steps of: (A) adding to the reaction mixture a high-activity, oil-soluble polymerization initiator, with a 10-hour half life temperature of no more than 40.degree. C. at a concentration of 0.1 mol/L in benzene, for a specified time within a period from the commencement of heat removal using the reflux condenser through to completion of the polymerization, (B) adding water either continuously or intermittently to the reaction mixture through the supply pipe for the polymerization initiator, at least during the period from the commencement of addition of the high-activity, oil-soluble polymerization initiator through to completion of that addition, and (C) passing steam through the supply pipe following completion of the addition of the water. According to the present invention, the process for producing a vinyl chloride-based polymer by aqueous suspension polymerization in a polymerization vessel fitted with a reflux condenser can be improved, the heat removal capability can be utilized effectively to shorten the polymerization time, and a high quality vinyl chloride-based polymer with an extremely low level of fish eyes can be produced.