PURPOSE: To utilize easily and economically combustion waste gas by introducing said gas to a gas separating apparatus incorporating a nonporous polymer membrane, concentrating, separating CO2, and using the separated gas for cooling bottom-blown multipipe nozzles of a converter.

CONSTITUTION: After a part of separated waste gas of a hot blast furnace is cooled through a gas cooler 1, the pressure is elevated by a blower 2 and the gas is introduced to a separator 3 incorporating a nonporous polymer membrane. Gaseous CO2 having high transmittance is transmitted through the polymer membrane to obtain a gas enriched with CO2, and the nontransmitted gas is released from the system through a pipe 4. Then, the pressure of the gas enriched CO2 is elevated by a compressor 6 via a vacuum pump 5 and used as a cooling gas for bottom-blown multipipe nozzles of a converter 7 by adding a part of high-purity gaseous CO2 procured on the market according to necessity.