PURPOSE: To obtain the subject resin having low hardness, reduced in compression set, and excellent in curability, etc., by reacting and foaming a mixture of an organic polyisocyanate with specific polyhydroxy compounds in the presence of water and a specific additive which serve as a catalyst, a foam stabilizer, and a foaming agent.

CONSTITUTION: A mixture of an organic polyisocyanate with polyhydroxy compounds containing 0.1-30wt.% polyoxyalkylene polyol which is obtained by polymerizing an alkylene oxide by ring-opening polymerization using a heterocyclic polyamine [e.g. N-(2-aminoethyl)piperazine] and has a mol.wt. of 150-2,500 per OH group is reacted and foamed in the presence of water and a given amount of an additive which serve as a catalyst, a foam stabilizer, and a foaming agent. The additive comprises at least one member selected from castor oils (derivatives) and compounds represented by formulae I to IV (wherein R and R1 each is a 1-30C hydrocarbon group; R2 and R3 each is a 2-4C hydrocarbon group; R4, R6, and R7 each is a 5-25C hydrocarbon group; R5 is a 1-15C hydrocarbon group; (k) is 1-60; (m) is 1-25; and (n) is 1-10).