To provide a method of optimizing combustion stability and heat efficiency of a combustion furnace while suppressing the increase of discharging quantity of NOx.

According to this invention, the discharge of NOx by combustion of coal in the furnace can be reduced. In this method, the furnace having a combustion chamber for burning coal by oxygen is used, and coal and metal-containing combustion catalyst are supplied to the combustion chamber with a small amount of oxygen in comparison with a case when the metal-containing combustion catalyst is not used. The heat efficiency and the combustion stability of the furnace are not impaired by reducing the supply of the combustion air and supplying a metal-containing additive to the combustion chamber.