PURPOSE: To further lower the NOx content of combusted exhaust gas without considerably altering a construction, by asymmetrically arranging a nozzle forming an air excess pre-mixing flame around a nozzle forming an air lean diffusing flame.

CONSTITUTION: In a burner comprising a nozzle 12 forming an air lean diffusing frame and a nozzle 11 forming an air excess pre-mixing flame provided close to the nozzle 12, an arrangement is altered such that the nozzle 11 forming the air excess pre-mixing flame is asymmetrically provided around the the nozzle 12 forming the air leans diffusing flame so as to halve the emission of NOx which would otherwise be produced due to a diffusing mixing between the air excess pre-mixing flame and the air lean diffusing flame.