According to a preferable embodiment of the present invention, a composition for a resin comprising an inorganic compound having a sulfur atom and/or a selenium atom, an episulfide compound, and a mercaptodisulfide compound can be provided. According to another preferable embodiment of the present invention, a composition for a resin having the viscosity thereof decreased to facilitate a cast polymerization operation and thus to improve the tonality of the obtained optical material can be provided. According to still another preferable embodiment of the present invention, an optical material obtained by curing the above-described composition for a resin and having a high refractive index can be provided.