PURPOSE: To reduce the number of constituent members and improve security, by moving a perforated mixing apparatus for mixing a gas fuel and air sucked with a gas stream along a longitudinal shaft core, and providing a sealing member between a mixing tube and an inner surface of the tube.

CONSTITUTION: An intake connection port 1 is connected with an intake end portion of a mixing tube 3, and an extrusion nozzle 4 is located at an exhaust end of the mixing tube 3, and further an inspiration tube 5 located coaxially with the mixing tube 3 is connected within the air intake connection portion 1. The inspiration tube 5 provides a final end 9 at a perforated mixing apparatus 2, and many small apertures 12 in the perforated mixing apparatus 2 are less than 1% of the size of an exhaust aperture. An annular sealing member 10 is provided between the perforated mixing apparatus 2 and an inner wall of the mixing chamber 3. The perforated mixing apparatus 2 is movable longitudinally to adjust a burner and hence to ensure a stabilized flame with optimum combustion. As a result, the adjustment of the burner in operation is simply ensured.