PURPOSE: To provide a stable combustion for the flame that is shortened and suppress the generation of CO by providing opposite to the internal end section of the burner duct of a supply air pipe, a rotating guide body that leads out the combustion air into the burner duct.

CONSTITUTION: The combustion air is once throttled by a throttling section 9a provided in a supply air pipe 9 and guided into a rotating guide body 30. Here, it collides with the central section of an opposing disc 31 and is dispersed circumferentially. And it is led out into a burner duct 6 from the gaps among many rotating vanes 33. Since each of the rotating vanes 33 is curved by the same radius of curvature and they are tilted mutually, the combustion air that is led out among those vanes 33 forms uniform rotating flows along all the circumference. The combustion air fills, therefore, uniformly in the inside of the burner duct 6, and it is supplied in a stable state from between a gas nozzle 21 and a throat 20 to a premixing chamber 19, and, at the same time, it is also supplied stably to secondary flame ports 13....