PURPOSE: To obtain a molded resin having a relatively high refractive index and suited as a lightweight spectacle lens material by subjecting a monomer mixture mainly consisting of t-butyl methacrylate and a tetrafunctional (meth) acrylic ester to free-radical cast polymerization molding.

CONSTITUTION: A mixture comprising 40-60 pts.wt. t-butyl methacrylate, 5-15 pts.wt. tetrafunctional (meth)acrylate of formula I (wherein X is a group of formula II or III) and 3-10 pts.wt. (meth)acrylic ester of formula IV (wherein Y is H or CH3; and R is 8-13C alkyl) and 15-50 pts.wt. comonomers is subjected to free-radical cast polymerization molding to obtain a lightweight molded resin having a refractive index of 1.50-1.52 and a resin density of 1.10-1.15g/cm3. This resin has a relatively high refractive index as compared with a molding made from a conventional spectacle material and is lightweight, and therefore it is useful as a material for forming plastic spectacle lenses.