To suppress separation to be generated by a top oxidation layer by adjusting the fuel jet momentum to be larger than the oxidant jet momentum to form a specified combustible mixture in a fuel-and-oxidant mixing layer with the oxidant and the fuel.

The oxidant momentum is adjusted to be below a threshold momentum by a valve 34. Actually, the speed of an underlay oxidant jet 28 is adjusted. When a fuel valve 36 is then adjusted, the speed and the momentum of a fuel jet 24 are adjusted. Since the momentum of the fuel jet 24 is adjusted to be larger than the momentum of the underlay oxidant jet 28, the fuel jet 24 generates a low-pressure area, the oxidant is kept away from a melt 10 and drawn toward the fuel to form a desired combustible mixture in a mixing layer of the fuel and oxidant. Since the separation generated by a top oxidation layer is suppressed, the output degradation by the flame can be prevented.