Disclosed is a process for producing chlorine by oxidation of hydrogen chloride through a catalytic gas-phase reaction in the presence of a catalyst, wherein hydrogen chloride as a raw material is a by-product of synthesis of isocyanate by reacting phosgene and primary amine and oxidizing the hydrogen chloride through a catalytic gas phase reaction in the presence of a catalyst. The process is characterized by adjusting the sulfur content in carbon monoxide which is a raw material for the phosgene to 2000 vol. ppb or less.