PURPOSE: To reduce the unit requirement of fuel in a heating furnace and improve the efficiency of recoverying a carbonic acid gas by utilizing oxygen which is generated at the time of the generation of hydrogen required for the synthetic reaction of an organic compound, and performing combustion under an oxygen-enriched state.

CONSTITUTION: The hydrogen generation side of an electrolyzing device 6 for water is connected to an organic compound synthesizing device 13. Further, the oxygen generation side of the electrolyzing device 6 for water is connected to an air pipeline 7 for feeding combustion air to a burner 2 via an oxygen pipeline 8, and oxygen-enriched air is fed to the burner 2. A material 4 to be heated is charged into a heating furnace 1. On the other hand, a carbon- containing fuel, which generates a carbonic acid gas by combustion, and an organic compound are supplied to the burner 2, whereby combustion is performed under an oxygen-enriched state. In this way, the unit requirement of fuel in the heating furnace can be reduced, and the concentrations of the carbonic acid gas and steam contained in a combustion gas become high.