converter 1 a large quantity of waste gas is produced during the middle period of the blow type smelting operation however the quantity of waste gas produced in the initial or final period thereof is relatively small when auxiliary raw material is put in the converter 1 or the flow rate of the oxygen gas jet through the pipe 3 is changed the quantity of waste gas produced also changes therefore in order to maintain the gas pressure in the hood 5 in a range of suitable gas pressures the flow rate of the waste gas is controlled in accomplishing this the gas pressure po in the hood 5 hereinafter referred to as 34 the pressure in the converter 34 or 34 the converter pressure 34 is detected the detected gas pressure value is applied to a controller 14 by a converter pressure signal generator 12 in the controller 14 the gas pressure value is compared with a predetermined value and an operating output signal is applied to a damper operating unit 15 so that the difference between the two values is zeroed as a result the opening and closing operation of secondary damper 9 is controlled whereby the flow rate of the waste gas is controlled in a closed type smelting.