PURPOSE: To prevent the deterioration of calorie in CO gas and the combustion of blown-out gas from skirt part by measuring N2 concentration in OG gas for a converter by gas analyzer and controlling the exhaust gas flowing rate in accordance with a deviation between the measured value and the setting value.

CONSTITUTION: Raw materials 2 are charged into the converter 1 and refining oxygen 4 is blown, and then carbon in the raw materials 2 is reached with the oxygen 4 and CO concentration in the OG gas 9 becomes to about 70W80%. When the pressure in the hood part 6 becomes to equal to the initial setting value (almost atmospheric pressure) and reaches a regular condition, three-way valve 20 is changed to recovery valve 21 side and controlling system is changed to automatic operation. The gas analyzer 42, such as mass spectrometer, etc., analyzes the sampling from the OG gas 9, to indicate as transmitting N2 concentration to a control device 44. By comparing this indicated valve of N2 concentration with the aimed value of N2 concentration beforehand set, control signal is transmitted to control valve 46 in accordance with the deviation, so as to become to zero for the deviation, and flow rate in the flue is controlled. And, in case of enclosing rather large quantity of air in the furnace by any cause, the control system detects immediately increase of N2 gas to shut the valve 46.