In the present invention, an episulfide compound having, in one molecule, at least one epithio structure represented by the following Formula 2: ##STR00001## wherein R.sup.5 is C1-C10 hydrocarbylene or single bond, R.sup.6, R.sup.7 and R.sup.8 are each independently C1-C10 hydrocarbyl or hydrogen, Y is O, S, Se or Te, p is an integer from 1 to 5 and q is an integer from 0 to 5, is polymerized in the presence of a halide of a 13-16 group element of the long periodic table and/or a polymerization regulator represented by the following Formula 1: ##STR00002## wherein R.sup.1, R.sup.2 and R.sup.3 are each C1-C10 hydrocarbyl or hydrogen, R.sup.4 is C1-C10 hydrocarbylene or single bond, X is F, Cl, Br, I, As, SH, OH, C1-C10 alkoxyl, C1-C10 alkylthio, or C1-C10 mercaptoalkylthio, and m is an integer from 1 to 5, to produce a cured resin suitable as an optical material. By using the polymerization regulator, the polymerization rate of the episulfide compound can be suitably regulated to obtain a cured resin with less defect. By using the halide, the increase in the viscosity of the starting composition during the storage or the casting operation can be prevented.