PURPOSE: To effectively control the rate of residual oxygen to get an ideal rate, by finding the rate of residual oxygen to the mixing ratio of both fuel from a compensating coefficient to the mixing ratio of both fuel and a suitability value in the aspect of residual oxygen of both fuel, in an industrial combustion furnace in which two kinds of fuel are used.

CONSTITUTION: When heavy oil and gas are mixed and burnt together, the flow rates of both oil and gas are detected by detectors 3 and 4, respectively, and a calorific value is compensated by compensators 6 and 7. Then, the mixing ratio of heavy oil to gas is found by a calculator 8, and a compensating coefficient to the mixing ratio is calculated by a function calculator 9. Both outputs from compensators 6 and 7 are added up by an adder 11, and a suitability value in the aspect of residual oxygen to the added value is calculated by a function calculator 10. Next, the rate of residual oxygen to the mixing ratio of both fuel is calculated by a multiplier 12 by multiplying the outputs from both function calculators 9 and 10, and the result is used as a setting value to a controller 14 to calculate the rate of oxygen. An air-fuel controller 16 is controlled by an output from the controller 14, by the intermediary of an arithmetic circuit 15 to compensate an air-fuel ratio.