To make it possible to lower the emission rate of a fuel gas by increasing a contact area between an emission gas flow and an air flow which provides an allowance to a primary air absorbing power and thereby reduce a pressure loss and enhance an absorption effect and prevent powder from being deposited on a gas nozzle or its support area by eliminating the need to support the gas nozzle in the central part of a mixing compartment.

A ring-shaped pressure compartment 4 in a peripheral wall 6 near the rear end of a cylinder-shaped mixing compartment which uses an opening 2 on the front end as a burner port and an opening 3 on the rear end as a primary air supply opening. A fuel gas is pressure-fed into the pressure compartment 4 while the pressure compartment 4 is communicated with the mixing compartment 1 by means of a ring-shaped slit 5 formed all over the peripheral wall of the mixing compartment 1 where there is provided a means which directs the flow of gas emitted from the ring-shaped slit 5 toward the front of the mixing compartment 1, thereby absorbing a primary air by the flow of emitted gas.