An optical resin prepared by curing a polymerizable composition comprising a (thio)epoxy compound having at least one intramolecular disulfide bond exhibits a considerably high refractive index while maintaining good optical properties and a high Abbe number. In addition, the above polymerizable composition to which is added primary and/or secondary amines as an yellowing inhibitor in a proportion of 0.001 to 0.5 of the total molar number of NH2 and NH groups in the amines to the total molar number of thioepoxy and epoxy groups in the (thio)epoxy compound having at least one intramolecular disulfide bond, can provide a transparent resin in which yellowing and reduction in heat resistance associated therewith are adequately prevented while maintaining a high refractive index.