PURPOSE: To permit the increase of heat exchanging efficiency, elimination of various kind of equipments, compacting and supplying stabilized heat by a method wherein operating liquid is sealed into the heat pipe and the capsule body, filled with heat accumulating agent, is accommodated and arranged in the heat pipe.

CONSTITUTION: When high-temperature discharging water is flowed into a high-temperature chamber 3, waste heat flows through the inflow port 4 to transfer the heat from the lower surface of the capsule 10, the operating liquid 13 of the heat pipe 12 is heated through the heat accumulating agent 11, the operating liquid 13 transfers the heat to the whole of the capsule 10 quickly, the heat is accumulated by the heat accumulating agent 11 and the low- temperature feed water in the low-temperature chamber 6 is heated through a partitioning plate 2. When the heat accumulating agent 11 of the side of the low-temperature chamber 6 is cooled by the heating of the feed water, heat is taken out uniformly from the whole of the capsule 10 by the operating liquid 13 of the heat pipe 12 to transfer the heat to the side of the low-temperature chamber 6. Then, the transportation of heat becomes irreversible whereby stabilized heat may be supplied within the range of heat accumulating capacity even when the input of heat is stopped, for example, during exchanging the heat and thus the heat accumulating heat transfer element 9, prominent in the heat exchanging rate and compact, may be obtained.