42 so as to form combustion chambers 43 and 44 which face each other vertically thus the upper combustion chamber 44 is communicated at one end thereof with the outside through an exhaust passage 45 and at one side end with an exhaust passage 48 defined by heat exchanger walls 46 and 47 39 the lower combustion chamber 43 is communicated with one end of a premixing chamber 47 since the other portions of this embodiment are similar to those of the first embodiment the corresponding portions are denoted by the same reference numerals and description thereof is omitted this embodiment enables the heat exchanger wall 46 of the premixing chamber 48 and the heat exchanger wall 47 39 of the air supply passage 28 to be efficiently and effectively heated by the use of only a part of the combustion gas while providing the same advantages as those of the first embodiment although a twin fluid atomizer fuel nozzle is employed in the foregoing embodiments an ultrasonic nozzle a swirl injection nozzle or the like may also be employed although in the foregoing embodiments preheating of air is effected by heat exchange with combustion gas by means of a heat exchanger preheating may also be effected by mixing air with a part of combustion gas referring next to fig3 which is a.