PURPOSE: To produce the titled foam excellent in mechanical properties and wet heat aging resistance, by reacting an organic isocyanate with a specified polyether-polyol in the presence of both of a reactive flame retardant and an additive flame retardant each containing P and/or halogen.

CONSTITUTION: The following compoents A, B, C and D are reacted in the presence of small amounts of a blowing agent, a catalyst and a foam stabilizer: (A) an organic isocyanate, (B) a polyether-polyol mixture, MW/OH group 300W2,000, consisting of a trifunctional polyether-polyol and 10W40wt% multifunctional (functionality ≥4) polyether-polyol, (C) 1W20wt%, based on total polyol, isocynato group-reactive, P and/or halogen-containing reactive flame retardant (e.g., dibromoneopentyl glycol) reactive with an isocyanate group and (D) additive flame retardant unreactive with isocyanate group contg. 0.5W1.5wt% P based on foam weight, and a halogen in an amount of 20W50 times (calculated as Cl) that of P.