PURPOSE: To prevent the production of short ingots at the ingot making and improve the yield of ingot making by decarbonization-refining the molten iron which has beforehand been prerefined of Si, S, P in a steel making furnace and making the required amount of ingot then adding the prerefined molten iron to the remaining molten steel, decarbonization-refining the same to the target C grade and making its ingots.

CONSTITUTION: Raw material molten iron for steel making is subjected to desiliconization, desulfurization and dephosphorization refining with Na2CO3, iron ore, oxygen, etc. by using a hermetic type groove type reaction vessel, whereby it is made to the molten iron of less Si, S and P. This molten iron is transferred into a steel making furnace such as converter, where it is subjected to decarbonization refining through oxygen top blowing refining until it becomes the target C grade, whereby it is made to steel. Of this, a fixed amount is weighed, is taken out and is made into ingots. In this case, it is only the decarbonization refining by oxygen and therefore slag is hardly produced. The molten iron having undergone prerefining is newly added into the molten steel remained in the converter and is decarbonization-refined to the steel of the target C grade, thence the required amount thereof is made into ingots. In this way, the steel may be taken out so as not to become odd in weight at the ingot making, the occurrence of short ingots is obviated and the yield of ingot making improves.