To reduce the oxygen concentration in the exhaust gas of a boiler to zero by completely burning a natural gas containing CH4 as a main component by using a small amount of fuel catalyst, by adding fuel having a lower combustion stating temperature than CH4 has to the natural gas as oxygen reducing fuel.

A boiler 1 is provided with a radiant evaporator 2 and a balance evaporator 3. The principal section of an oxygen reducing device comprises a combustion catalyst section 11 and a fuel supplying device 12, and the oxygen concentration in the exhaust gas of the boiler 1 is reduced to ≤1,000 ppm through secondary combustion in the section 11. The catalyst used in the section 11 contains a metal of the platinum group, such as platinum, etc., or the oxide of aluminum, etc., as an active component. CH4 is a hardly burned combustible gas, because the combustion starting temperature of CH4 is 370-380°C when the catalyst is used. When H2, etc., is added to a natural gas containing CH4 as a main component, therefore, the combustion starting temperature of the gas can be lowered and oxygen can be removed surely from the exhaust gas of the boiler 1 by means of the oxygen reducing device.