PURPOSE: To enable to obtain stable fire flames in either case of oil burning and gas burning by installing nozzles in such a manner that gas nozzles and premixing air nozzles in a mixing tube are installed in the same circle and those nozzles are arranged at equal distances and with zigzag form so as to be easily changed by proper means.

CONSTITUTION: At the periphery of a diffuser part 6 a mixing tube 9 connected to gas tubes 8 branched from gas manifolds 7 is installed, and gas nozzles 10 secured to connecting parts with gas tubes and premixing air nozzles 11 are installed in the same circle at the mixing tube 9. The gas nozzles 10 and the premixing air nozzles 11 are arranged at equal distances and with zigzag form in the same circle, and the gas nozzles 10 and the premixing nozzles 11 are easily changed by such means as screw fitting, etc. in the mixing tube 9. Thereby, as fuel and air are satisfactorily mixed, fire flames are made stable in either case of oil burning and gas burning and also if gas nozzles are changed, the nozzles are applied to any type of gas.