PURPOSE: To perform an efficient recovering of heat from heat gas within a short period of time by a method wherein a thin-walled solid material is used as a heat accumulation material and a total heat capacity of fluid with respect to heat gas flowing within a heat accumulation layer composed of the heat accumulation material is more than an approximate equal amount.

CONSTITUTION: Heated heat accumulation material is present within a heat accumulation chamber 2, air is flowed within the heat accumulation chamber 2 from a lower part to an upper part, heat of the heat accumulation material is transmitted to the air and absorbed there. Since the heat accumulation material is of a thin walled material, heat of the heat accumulation material is efficiently transmitted within a short period of time. In this case, a total calorie of air supplied into the heat accumulation chamber 2 by operating an adjusting valve 13 is adjusted to be substantially equal to or more than a total heating capacity of the heat gas supplied to the heat accumulation chamber. Accordingly, an approximate total amount of heat transmitted to the heat accumulation material with the heat gas is absorbed into the air, a temperature of the heat accumulation material in an entire region of the heat accumulation chamber is decreased and then a heat absorbing amount when the heat accumulation material is heated by heat gas is increased. The heated air passes through the valve 8 to heat an air filling layer 3 of a distribution box and to heat feed water to the auxiliary boiler 4 and then the air is discharged through a discharging line 41.