Disclosed is a polymerizable composition comprising Component (a): a polythiocarbonate polythiol, Component (b): an episulfide compound, and optionally Component (c): an organic compound having a functional group that is reactive to the episulfide compound. The polymerizable composition can be cured to provide a polythioether having excellent optical properties (a high refractive index and a high Abbe's number) as well as excellent mechanical properties (a high bending distortion and a high glass transition temperature) and thus exhibiting excellent properties as an optical material.