Abstract: The reactive injection molding of a urethane-based resin composition according to the invention is performed by admixing the polyol compound as one of the reactants of the urethane-based resin with a specific organopolysiloxane as an internal mold release agent so that the molding process can be performed without application of an external mold release agent to the metal mold. The organopolysiloxane is characterized by two types of substituent groups bonded to the silicon atoms, of which the first type is --C.sub.x H.sub.2x OR', R' being a monovalent hydrocarbon group of 8 to 30 carbon atoms free from chain end unsaturation and x being 0-4, and the second type is --C.sub.y H.sub.2y (OC.sub.n H.sub.2n).sub.m OH, y being 1-4, n being 2-4 and m being 0-45.