PURPOSE: To realize wide operation area in the minimum exhaust gas emission, by arranging large and small premixed burners alternately along the entire front wall of a combustion chamber, and providing an air nozzle at every time.

CONSTITUTION: A main burner B and a pilot burner C which are arranged alternately as communicate with each other at the same height in an integral front wall 10 like a ring constituting the entry surface of an annular combustion chamber A. Several air nozzles D are provided between the burners B and C at every time, and the air nozzle occupies an almost half of front wall 10 in the radial direction. The center of small vortex 15 at a high temperature around the pilot burner C, and it is located between the center and the center of large vortex at a comparatively low temperature due to the main burner B. The air nozzle works in an area therebetween. Further, it improves the stability of both vortexes definitely.