PURPOSE: To increase thermal efficiency, reduce heat transfer area and make the device compact by increasing temperature difference between high-temperature side and low-temperature side.

CONSTITUTION: An evaporating pipe group 7 is arranged in a waste gas duct 2 for high-temperature waste gas 1 (heating medium), first condensing tube group 10 is arranged in an air duct for combustion air 3 (medium to be heated) and the second condensing tube group 13 is arranged in a fuel gas duct 6 for fuel gas 5 (medium to be heated) while a part of the evaporating tube group 7 forms first closed loop type enclosed vessel by first upper and lower connecting tubes 16, 17 through headers 8, 9, 11, 12 and the balance of the evaporating pipe group 7 forms the second closed loop tye enclosed vessel by the second upper and lower connecting tubes 18, 19 through the headers 8, 9, 14, 15. In such manner, the first and second closed loop type enclosed vessels are arranged alternately, therefore, the temperature difference between the high-temperature side and the low-temperature side may be increased the efficiency of a heat exchanging rate may be increased.