Rigid polyurethane foams produced by reacting a polyol component having a special composition with an isocyanate component in the presence of a blowing agent, a reaction catalyst and a foam stabilizer with a mold release time of as short as 5.5 minutes or less, have remarkably low thermal conductivity of 12.5.times.10.sup.-3 Kcal/m.hr..degree.C. or less and a low density and are suitable as heat insulating materials for refrigerators and the like.