A fuel being primarily a liquid fuel is ejected through a fuel ejection pipe having an annular ejection opening; primary air is ejected through primary air-ejecting openings arranged on outer and inner sides of the fuel ejection opening, to form outer and inner primary air-ejection straight stream openings between which the fuel ejection stream is interposed, and to burn the fuel ejection stream. The fuel-spraying openings are arranged on the same circumference. The outer and inner primary air-ejecting openings, respectively, are each arranged on a respective concentric circumference having a center point identical to the center of the circumference on which the fuel-spraying openings are arranged. When a power fuel is used, optionally a liquid fuel is further ejected and mixed with the primary air stream, and the liquid and power fuels are burnt together.