Disclosed is a polythiocarbonate poly(thio)epoxide in which a hydrogen atom in a mercapto group of a polythiocarbonate polythiol is substituted with a (thio)epoxy-containing group. By using this compound as a starting material, there can be obtained a polythioether or a sulfur-containing polyether having, in addition to excellent optical properties (a high refractive index and a high Abbe's number), excellent mechanical properties (a high bending distortion) and a high glass transition temperature.