A urethane-modified polyisocyanurate foam obtained by reacting (A) a polyisocyanate compound component, (B) a polyol component, containing a modified phenolic resin obtained by adding 20 to 100 parts by weight of a polyhydric alcohol or its alkylene oxide adduct to 100 parts by weight of a benzylic ether type phenolic resin and heating under a reduced pressure, in an amount of at least 3 wt % to the total resin component, (C) water and (D) a foam stabilizer comprising a mixture of at least 2 silicone type surfactants having different surface tensions wherein a surfactant having a higher surface tension has a surface tension of higher than 22 dyne/cm and a surfactant having a lower surface tension has a surface tension of at most 22 dyne/cm, in the presence of (E) a urethane-forming catalyst and/or a trimerization catalyst.