The invention relates to a curable composition preferable as a raw material for optical materials such as adhesive agents for optical elements, coating agents for optical elements, resist materials, prisms, optical fibers, information recording substrates, filters and plastic lenses, and more specifically to a curable composition characterized by containing (A) an episulfide compound, and (B) a polymerization catalyst comprising a tetraalkylphosphonium dialkylphosphate represented by the following general formula (1): wherein R1 to R6 are the same or different, an alkyl group having 1 to 8 carbon atoms or an alkyl group having a hydroxyl group and 1 to 8 carbon atoms, and linear, branched or cycloaliphatic.