PURPOSE:To permit the control of the percentage of O2 in exhaust gas without detecting it directly, by a method wherein an optical power signal is detected from a flame and the optical power signal is processed to obtain a control output. CONSTITUTION:An optical sensor 15, converting an optical power emitted from the combustion flame 14 of a burner 4 into an electric signal, is fixed to a place opposing to the flame 14. Optical power level in a minute time is taken into an operator 18 from an A/D converter 17 converting the analog signal from the light sensor 15 into a digital signal and the average value during (t) seconds of the average value during (DELTAt) seconds is operated to operate the integrated value Y of deviation. Subsequently, a difference B between an objective percentage of O2 is obtained by the integrated values A, Y of deviations in a regulator 19 to output the correcting value of air. This output is inputted into a regulating valve 9. Thus, exhaust gas, generated in the main body 1 of a furnace, is regulated so as to contain the predetermined optimum percentage of O2 and is discharged to the outside of a system through a flue 3 while low O2 combustion may be achieved in the furnace.