A polymerization apparatus, comprises a polymerization vessel, a reflux condenser installed outside the polymerization vessel and a pipe connected between the reflux condenser and a wall of the polymerization vessel with an end thereof opening into a gaseous phase region inside the polymerization vessel, for returning the liquid condensate to the polymerization vessel, wherein said end projects from an inner surface of the wall of the polymerization vessel into the gaseous phase region. In polymerizing a monomer having an ethylenically unsaturated double bond using the polymerization apparatus, the quantity of heat removed by the reflux condenser is increased stepwise or continuously with progress of the polymerization, such that not less than 20% of the total reaction heat generated during the whole course of the polymerization is removed by the reflux condenser. With 100 or more repeated polymerization runs, polymer scale deposition inside the vessel is minimal even below the pipe end.