A polymerization reaction apparatus is used for producing a polymer by photo-polymerizing a polymerization precursor by irradiation with an active energy ray in a supercritical fluid or subcritical fluid. The apparatus includes a reaction vessel (3), active energy ray irradiation means (6, 5), a fluid feeding means of feeding a fluid into the reaction vessel, and a fluid discharging means of discharging the fluid. The apparatus further includes [1] polymerization precursor storage parts (11, 11') for accommodating a polymerization precursor and/or addition components to be contained in the polymer and feeding the polymerization precursor and/or addition components to a reaction field, or [2] a polymerization precursor feeding means capable of feeding a polymerization precursor and/or addition components into the reaction vessel.