A polymerization apparatus comprising a cylindrical polymerization vessel having a straight cylindrical section, a cooling means for cooling the contents of the polymerization vessel, and paddle impellers, each fixed to a central shaft positioned along the center line of the vessel and spaced apart at predetermined positions, each impeller having impeller blades for agitating the contents inside the polymerization vessel as the shaft is rotated, wherein the polymerization vessel has an inner capacity of 100 m.sup.3 or more and has a ratio L/D (the length L of the cylindrical section to the inner diameter D of the vessel) of at least 1.5, wherein three or more of the paddle impellers are arranged on and along said shaft in the center of the vessel, and wherein polymerization/inhibitor charge openings are arranged at positions on the interior surface of the wall of the polymerization vessel in such a manner that one charge opening is provided per paddle impeller at a position on the vessel wall which corresponds to the position of a paddle impeller along the shaft.