PURPOSE: To reduce consumption of Ar gas and to maintain the suitable N2 concentration in stirring gas by diluting CO gas containing N2 by the other stirring gas in case of exceeding the prescribed value of N2 concentration in the CO gas for using bottom blowing stirring gas.

CONSTITUTION: A converter 1 has a construction, by which CO, CO2 and Ar gas are blown into a molten steel 2 from tuyeres 4 at the bottom part thereof. The CO gas is separated from the converter gas by de-CO gas device 7 and contains N2 as impurity. The N2 concentration in the CO gas is calculated by an arithmetic unit 22 from an outputting signal of analyzer 18 arranged in the pipe 8. And, in the arithmetic unit 22, in case of being the allowable limit value (L) for N2 concentration fixed by steel kind, etc., or less, the opening signal is outputted only to a value 14, and CO gas is supplied into the furnace 1. In case of exceeding the N2 concentration from the value L, the signal is outputted to valves 14W16 and each opening degree is adjusted, and CO gas is diluted by CO2 and Ar gas, to maintain the concentration at the value L or less.