PURPOSE: To achieve economical refining of steel without denitration-treatment by following method in which CO gas generated in the maximum decarburization period is directly drawn out from the inner mid portion of an oxygen converter, and is brown into the molten steel bath from the converter-bottom.

CONSTITUTION: At the muximum decarburization period of an oxygen refining furnace such as a converter 1, generated CO gas is directly drawn out by using a drawing lance 6 and is accumulated in a holder 8. The nitrogen content in the atomophere in a converter 1 is below 1% in said maximum decarburization period, therefore the generated gas is almost pure CO gas. This pure CO gas is brown into the molten steel bath from the converter-bottom by way of a regulator 9 for controlling flow amount of gas. Accordingly, pure CO gas is directly brown into the steel bath without denitration treatment by this process.