PURPOSE: To conduct combustion exhaust gas recirculation effectively by utilizing exhaust kinetic energy of combustion exhaust gas exhausted from a radiant tube.

CONSTITUTION: An outlet portion 11b of a U-shaped radiant tube 11 on the side of an exhaust cylinder 12 is formed in the shape of a funnel, with the exhaust passage aperture gradually reduced, and one end of a recirculation pipe 8 is inserted into the reduced-aperture portion 11b. After the combustion exhaust gas of the fuel jetted from a burner 1 and burnt is increased in exhaust kenetic energy at the outlet portion 11b, a portion of the exhaust gas is flowed into a header portion 9 mainly by its own kinetic energy, flowed through the recirculation pipe 8, and recirculated from a nozzle portion 10 to the burner 1 automatically. Thus, the recirculation of exhaust gas can be conducted properly by only setting a regulator valve 15, without use of a special mechanism such as suction fan.