PURPOSE: To facilitate the separation of moldings from molds, by a method in which a polyol, an organic isocyanate, a catalyst, and an organopolysiloxane having a specified structure as a releasing agent are mixed, and the mixture is cast into molds and hardened.

CONSTITUTION: A polyol, an organic isocyanate, and a catalyst are mixed with an organopolysiloxane consisting of 99.5W80mol% siloxane unit of formula III(where R4 is a monovalent hydrocarbon and c is 0W3 on the average) and 0.5W20mol% siloxane unit of formula I [where R is a group of formula II(where R2 is an alkylene, R3 is a divalent hydrocarbon, m is an integer of 2W30, and n is an integer of 1W5), R' is a monovalent hydrocarbon group, a is 1W3 on the average, and b is 0W2 on the average]. The mixture is cast into molds of a desired shape and hardened by reaction to obtain a reaction injection molding.