as sodium sulfide or potassium hydroxide for the heat accumulating liquid and which accumulates the heat by utilizing the reversible thermal chemical reaction based upon the heat that is generated when the heat accumulating liquid is decomposed or is formed the medium used in this method may be an aqueous solution of magnesium chloride or the like however use can also be made of a dilute aqueous solution of sodium sulfide or potassium hydroxide for the heat accumulating liquid an embodiment will be described below with reference to a concretely constructed apparatus according to the invention a heat accumulating liquid tank 550 has a shape which is the same as that of an absorbing medium tank 150 and their size changes depending simply upon the amount of liquid to be stored the construction will be described hereinbelow with reference to the tank for storing heat accumulating liquid fig3 a is a vertical sectional view of the tank for storing heat accumulating liquid employing tubular porous materials porous pipes 1000 and fig3 b is a horizontal sectional view thereof in a vessel 560 are vertically arranged many heat transmission pipes 610 which are collected together at their lower portions by a manifold 620 and which are connected at their upper portions to an inlet liquid chamber 640 and to an outlet liquid chamber 630 many porous pipes 1000 are arranged among the heat transmission pipes 610 as shown in fig3 a b the lower portions thereof being closed and.