Disclosed is a polythiourethane having a high refractive index and low dispersion which is obtained through a reaction between a polythiol component containing one or more polythiocarbonate polythiol (preferably one having a —(CO—S—R—S)— structure (wherein R represents a divalent hydrocarbon group) and a number average molecular weight of 200-2,500) and a polyisocyanate component containing a polyisocyanate and/or a polyisothiocyanate. This polythiourethane is useful for plastic lenses, prisms, optical fibers, substrates for information recording, colored filters, infrared absorbing filters and the like.