A method for reducing nitrogen oxide(s) and carbon monoxide from flue gases of an industrial burner, said method comprising the steps of: a) providing an industrial burner adapted to burn gaseous and/or liquid fuel, and comprising a burner automation containing measuring instruments, and a mixing zone accompanied by a combustion chamber, said burner automation being in communication with measurement instruments of flue gas conduit, said combustion chamber or flue gas conduit being provided with two successive three-way catalytic converters (401,401) with catalytic zones (40, 40) which catalytic zones (40, 40) are successive in progressing direction of flue gases, said two successive three-way catalytic converters comprising a first three-way catalytic converter comprising a catalytic zone and a second three-way catalytic converter comprising a catalytic zone; b) delivering an inlet flow (Q l, Q ltot) of combustion air and an inlet flow (Q PA, Q PAtot) of fuel into the mixing zone; c) generating flue gases in the combustion chamber by combusting air and fuel delivered into the mixing zone in step b); d) measuring the amount of residual oxygen, in flue gases by measuring the flue gas oxidation/reduction potential by a lambda-sensor; e) directing the flue gases to said at least one three-way catalytic converter(s); f) adjusting the inlet flow (Q l, O ltot) of combustion air and the inlet flow (Q PA, Q PAtot) of fuel arriving in the mixing zone in step b) by means of said burner automation, so that the mean amount of residual oxygen in moles compared to mean amount of carbon monoxide in moles, is 0.5/1 (mole /mole) and O 2 within the range of 0.01-0.50 vol-% preferably in the range of 0.01-0.25 vol % in flue gases prior to first catalytic zone or at said first catalytic zone of said two three-way converters; g) delivering supplementary air between catalytic zones (40, 40) of said two successive three-way catalytic converters, so that concentration in the flue gases after said catalytic zones (40, 40) is within range of 0-9 ppm for NOx and within a range of 0-100 ppm preferable in the range of 0-40 ppm for CO. a) providing an industrial burner adapted to burn gaseous and/or liquid fuel, and comprising a burner automation containing measuring instruments, and a mixing zone accompanied by a combustion chamber, said burner automation being in communication with measurement instruments of flue gas conduit, said combustion chamber or flue gas conduit being provided with two successive three-way catalytic converters (401,401) with catalytic zones (40, 40) which catalytic zones (40, 40) are successive in progressing direction of flue gases, said two successive three-way catalytic converters comprising a first three-way catalytic converter comprising a catalytic zone and a second three-way catalytic converter comprising a catalytic zone; b) delivering an inlet flow (Q l, Q ltot) of combustion air and an inlet flow (Q PA, Q PAtot) of fuel into the mixing zone; c) generating flue gases in the combustion chamber by combusting air and fuel delivered into the mixing zone in step b); d) measuring the amount of residual oxygen, in flue gases by measuring the flue gas oxidation/reduction potential by a lambda-sensor; e) directing the flue gases to said at least one three-way catalytic converter(s); f) adjusting the inlet flow (Q l, O ltot) of combustion air and the inlet flow (Q PA, Q PAtot) of fuel arriving in the mixing zone in step b) by means of said burner automation, so that the mean amount of residual oxygen in moles compared to mean amount of carbon monoxide in moles, is 0.5/1 (mole /mole) and O 2 within the range of 0.01-0.50 vol-% preferably in the range of 0.01-0.25 vol % in flue gases prior to first catalytic zone or at said first catalytic zone of said two three-way converters; g) delivering supplementary air between catalytic zones (40, 40) of said two successive three-way catalytic converters, so that concentration in the flue gases after said catalytic zones (40, 40) is within range of 0-9 ppm for NOx and within a range of 0-100 ppm preferable in the range of 0-40 ppm for CO.