PURPOSE: To effectively transmit the heat of heat accumulating material to heat transfer medium by a method wherein the evaporation and condensation of working liquid is performed in cycle.

CONSTITUTION: In order to draw heat out of the device, low temperature heat transfer medium is fed into a piping 6. Thereupon, the vapor of the working liquid 3 releases through a heat exchanger 4 its heat and condenses into liquid. At that time, the vapor pressure in the upper vapor phase part lowers and said lowering of the pressure is supplemented by the vapor of the working liquid 3. At this time, vapor bubbles 10 agitate the heat accumulating material 2, resulting in making the temperature distribution within the heat accumulating material 2 uniform. Furthermore, since the specific gravity of the condensed working liquid 3 is larger than that of the heat accumulating material, the condensed working liquid 3 precipitates. However, a part of the precipitating working liquid 3 absorbs heat from the heat accumulating material 2 and evaporates again so as to ascent. While, the working liquid 3 precipitated onto the bottom surface of the heat accumulating tank 1 absorbs heat from the surroundings and evaporates itself and turns into the bubbles 10 in order to ascend so as to attain to the heat exchanger 4. On the other hand, the low temperature heat transfer medium fed through the piping 6 receives heat at the heat exchanger 4 and turns to high temperature heat transfer medium to be flowed forth from a piping 7. On the contrary, in order to accumulate heat in the device, it is enough to feed high temperature heat transfer medium from a piping 8 so as to heat-exchange at a heat exchanger 5.