Deoxidizing and purifying action of slag on molten steel during the reduction period is accelerated by using fluorspar and limestone in 1 : 1 proportion thus rendering the slag “super-basic” and throwing in cokes to form it into a carbide slag. The feature of this method lies in the fact that the disadvantages accompanying high temperature melting as in the usual method are not experienced, therefore, the higher the temperature, the more complete and rapid becomes the deoxidizing and purifying action of the slag. Further, the method is featured by the unexpectedly low damage to the furnace lining.