PURPOSE: To obtain a polyurethane foam having improved compression strength without any adverse effect on its thermal insulation characteristics.

CONSTITUTION: In the rigid polyurethane foam which is obtained by reaction of a polyisocyanate with a polyol in the presence of a catalyst and a foam stabilizer, a mixture of HCFC-141b and water is used as a foaming agent, a trimerization catalyst is used as a catalyst and the reaction is carried out under such conditions as the isocyanate index is kept less than 150.