To save a space and increase the degree of freedom in arrangement by providing an air box for mixing combustion gas and air for combustion at one side end in the longitudinal direction of a rectangular casing and turning a passage for the combustion gas in the casing.

Combustion gas 25 is previously mixed with air 26 for combustion in a mixing air box 2 and the mixed air is burnt by a face combustion burner 6. High temperature combustion gas is transferred to first heat transfer pipes 9 in a first gas passage 8 and cooled, so that a high temperature residence time is decreased and the production of nitrogen oxides is suppressed. The combustion gas is guided to a heat insulating space 10 to completely burn carbon monoxide. Then, a combustion gas flow is changed by 90°, the heat of the gas is transferred to second heat transfer pipes 12 and heat transfer pipe walls 14 and 16 while the gas flows in a second gas passage 11 and further cooled. Subsequently, at the end of the second gas passage 11, the flow is changed by 180°, enters a third gas passage 17 and the heat of gas is transferred to heat transfer pipe walls 16 and 19 and cooled. Further, the flow is changed again by 180° at the end of the third gas passage 17, the heat of gas is transferred to heat transfer pipes 24 for low temperature and the heat transfer wall 19 through a fourth gas passage 20 and the gas is exhausted from an exhaust gas outlet 21.