PURPOSE: To eliminate necessity of air fans and thus simplifying equipment by adding air branched from an air compressor into oxygen-containing high- temperature exhaust gas from a gas turbine and supplying a gas mixture thus obtained to a late-stage furnace or a hearth for oxygen-containing combustion.

CONSTITUTION: A portion of the air supplied into an air compressor 1 via a conduit 2 is guided outside via a conduit 18 after a first stage compression in the air compressor 1 and added to exhaust gas guided from a gas turbine 7 via a conduit 9. A gas mixture thus formed is delivered as the gas for oxygen- containing combustion to pyrolysis furnaces A to F through a conduit 12. Further, an air flow branched from the air compressor 1 is further rendered to flow through an heat exchanger 19 disposed in midway of the conduit 18, so that it can be warmed before its mixing with the exhaust gas from the gas turbine 7 through the conduit 9. Therefore, equipment cost can be reduced due to simplification of devices without needing additional air fans.