Abstract: A liquid-state resin composition containing an aromatic polyisocyanate compound (A), a bisphenol epoxy resin (B) and an imidazole compound (C) is described, wherein the molar ratio of isocyanate groups to epoxy groups is 2 or more and preferably 2 to 15, and the amount of (C) relative to the total weight of (A), (B) and (C) is in the range of 0.2 wt % to 0.8 wt %. By stir-mixing (B) and (C), adding (A), stir-mixing again, and vacuum-degassing the resulting resin composition, a highly thermo-resistant isocyanurated cured material with a glass transition temperature of 250° C. or higher can be obtained by heat-curing the resin composition, without forming a gelated substance or bubbles.