A resin having a low specific gravity, an excellent impact resistance, and a high refractive index was produced by radical polymerization of a mixture of difunctional methacrylate, such as tetraethylene glycol dimethacrylate or triethylene glycol dimethacrylate, styrenic monomer such as styrene or a-methyl styrene, and acrylonitrile. We provide a totally balanced optical material composed of the said resin, especially a lens.