Abstract: A method for producing a poly(vinyl chloride) comprises polymerizing vinyl chloride or a mixture of copolymerizable monomers containing vinyl chloride as the main component, in an aqueous medium, wherein the polymerization is carried out in a reaction vessel which is formed by arranging partition plates at certain intervals at right angles to the outer surface of an inner barrel and overlaying outer strips on and between the free ends of the partition plates to thereby form a flow passage unit having a flow passage for a heating medium defined by the partition plates, the inner barrel and the outer strips ("a temperature control element"); fixing the temperature control element in a vessel body so as to oppose the side of the outer strips with a gap to the inner surface of the vessel body, and sealing the upper and lower portions of the gap formed between the outer strip side of the temperature control element and the inner surface of the vessel body to form a gap chamber.