PURPOSE: To improve the stability of combustion, and to realize sufficient high-load combustion and low NOx combustion by forming flame holes positioned on the outer circumferential side in flame holes in a shape that diameters discontinuously change in small diameters on the inlet sides and large diameters on the outlet sides and forming flame holes positioned on the center side in equal diameters.

CONSTITUTION: A gas fuel feed path 3 and a combustion-air feed path 4 are connected to a gas feed chamber 2 in a burner case 1, and a large number of flame holes 6, 7 are formed under the state in which they are in communication with the same gas feed chamber 2 by a flame-hole forming member 5 installed in the burner case 1. The flame holes 6 located on the outer circumferential side in the flame holes are formed in a shape that diameters discontinuously change in small diameters on the inlet sides and large diameters on the outlet sides, and the flame holes 7 located on the center side are formed in approximately equal diameters extending over overall length. Gas spray velocity from the fame holes 7 is increased, gas spray velocity from the flame holes 6 is reduced, a flame is held by the action of stepped sections 6a in the flame holes 6 on the outer circumferential side, and one flame is formed stably by a flame hole group even in high-load combustion and low NOx combustion at a large air ratio.