PURPOSE: To obtain a heat accumulator which can cope with reaction of a high pressure of a heat accumulation material and in which its heat accumulation amount can be freely selected by forming a heat reservoir of a platelike unit type.

CONSTITUTION: A heat accumulation material 11 and a heat transfer medium 12 are filled in a heat accumulation plate 11 formed with a hollow part. Heat exchangers 6, 10 are arranged in the material 11 and the medium 12. That is, the exchanger 6 for chilled water is arranged in an upper part in the plate 1, the exchanger 10 for heating water is arranged in a lower part, and the exchangers 6, 10 are coupled in series with the adjacent plate 1. For example, the three plates 1 are coupled to constitute one set of a heat accumulation unit, three sets of the units are clamped by a band plate 2 and contained in a case 13 formed of a heat insulator. Thus, a heat accumulator which can cope with reaction of a high pressure of the material and in which its heat accumulation amount can be freely selected can be obtained.