PURPOSE: To control easily the suction rate of waste gas with a continuous steel making furnace by providing a control circuit which controls the suction rate of the waste gas in accordance with the change in the rate of oxygen refining.

CONSTITUTION: An air flow control damper 21 is provided in the midway of a discharge duct 20' for waste refining gas and the degree of opening is adjusted by a servocontrol motor 28 to control the suction rate of waste gas. The amt. of pure oxygen 18 supplied to anoxygen lance 13 is detected by an orifice 25 and is indicated on an indicator 26. A control circuit which is provided with a controller 27 and controls automatically the degree of opening of the damper 21 by a servocontrol motor 28 upon receiving of a change in the supply rate of oxygen as input is provided to such discharge section. The suction rate of the waste gas is thus controlled in accordance with the rate of oxygen refining. As a result, the inside of the refining furnace is maintained roughly under the specified pressure condition and the refining furnace is stably operated.