<p>Disclosed is a polythiocarbonate poly(thio)epoxide obtained by substituting a hydrogen atom in a mercapto group of a polythiocarbonate polythiol with a (thio)epoxy group-containing group. By using this compound as a raw material, there can be obtained a polythioether or a sulfur-containing polyether having an excellent mechanical property (high strain allowance against bending) and a high glass transition temperature in addition to excellent optical properties (high refractive index and high Abbe number).</p>
