PURPOSE: To simplify a device and improve the reliability of switching by a method wherein two passages are defined by a partitioning plate and an air introducing unit as well as an exhaust gas discharging unit are connected to one side of both passages while both passages are provided with a heat accumulating body, a flame keeping unit and a U-turn unit and a gas nozzle is connected to the heat accumulating body.

CONSTITUTION: An air introducing unit 2, communicated with respective passages A, B, is formed at one side of the passages A, B defined by a partitioning plate 1 while an exhaust gas discharging unit 3, communicated with respective passages A, B, is formed so as to be neighbored to the downstream side of the air introducing unit 2. The air introducing unit 2 is provided with an air switching valve body 4 and the exhaust gas discharging unit 3 is provided with a swing valve 5 while heat accumulating body 6, to which a gas nozzle 10 is connected, is provided in the passages A, B at the downstream side of the swing valve 5. Further, the heat accumulating body 6 is provided with a flame keeping unit 7 at the downstream side thereof while both passages A, B are communicated with a U-turn unit provided at the downstream side of the flame keeping unit. In this device, ignition is effected by one piece of pilot burner 9 provided on the partitioning plate 1 whereby the simplification as well as the cost reduction of the device are contrived.