PURPOSE: To reduce the generation of NOX, by reducing NOX produced by primary combustion of fuel by use of a catalyst.

CONSTITUTION: When a mixture gas in a primary combustion chamber 13 is ignited by an igniting means, primary combustion of fuel gas is caused by air supplied at a rate smaller than that required theorectically, and a small amount of NOX such as NO and NO2 is produced by the primary combustion. When the primary combustion gas containing the NOX as well as CO, H2 gases, etc. is introduced into a secondary combustion chamber 14 through a partition 12, NOX is reduced by an NOX reducing catalyst constituting the partition 12. With such a simple arrangement of a burner, it is enabled to reduce the generation of NOX.