A process in which a polyurethane/thiourethane resin can be produced from a polyisocyanate compound, a polyol compound, and a polythiol compound without causing abrupt heat generation or viscosity increase. At least one polyol compound having one or more ether bonds and two or more hydroxy groups per molecule is used as the polyol compound, whereby large heat generation and an abrupt viscosity increase are inhibited from occurring upon monomer mixing.