To provide a sulfer removal method for a used slag, by which the steelmaking excellent in quality can be performed by reusing the used slag increased in sulfur content as the desulfurizing slug satisfactory in desulfurizing capacity.

In the steelmaking process, the used slag 4 increased in the sulfur content by being subjected to the desulfurizing treatment, is held in a heat-treatment furnace B and the inner part of this furnace is made into the carbon dioxide atmosphere and is heated to 900C for 1 hrs, thereby removing the sulfur component without changing the other components other than the sulfur component from the used slag to increase the desulfurizing capacity of the used slag 4, so that the slag can be reused.