PURPOSE: To produce an open-celled foamed synthetic resin having a high dimensional stability without causing the ozone destruction problem by reacting a specific active hydrogen compd. component with a polyisocyanate compd. in the presence of water as the blowing agent, a catalyst and a silicone foam stabilizer.

CONSTITUTION: A component comprising 20-90wt.% active hydrogen compd. with a mean functionality of 4 or higher and a mean hydroxyl value of 300 mgKOH/g or higher and 80-10wt.% active hydrogen compd. with a mean functionality of 4 or lower and a mean hydroxyl value of 200mgKOH/g or lower is used as the active hydrogen compd. component and water in an amt. of 3-10wt.% of the above component is used as the blowing agent in the process for producing a rigid foamed synthetic resin by reacting the active hydrogen compd. component comprising active hydrogen compds. each having at least two isocyanate-reactive active hydrogen groups with a polyisocyanate compd. in the presence of the blowing agent, a catalyst, and a silicone foam stabilizer. Thus, it is obtd. an open-celled foamed synthetic resin with a high dimensional stability while using water, which releases carbon dioxide, as a blowing agent not destroying the ozone layer.