PURPOSE: To increase the thermal efficiency of an underground heat accumulator and reduce the installation cost thereof by a construction wherein the under- ground heat exchanger is formed in regular square or regular polygon or a shape approximate to it.

CONSTITUTION: An underground heat accumulator which comprises a heat exchanger 2 buried underground and which is connected with a solar heat collector 5 to form an underground heat accumulation zone 3 has a regular square or regular polygon or a shape approximate to it in the overall plane conufiguration thereof. In case the ratio N of length L to width W of the heat exchanger 2 is 1, i.e, it has a regular square shape, the effective heat flow ratio qv/qt (wherein qt is a total heat flow (Kcal/hr) created by the heat exchanger 2, qv an effective heat flow (Kcal/hr) transmitted to the ground under the floor of a building 1) becomes its maximum value. The effective heat flow ratio reduces with increase in the value of N. Further, in case all the sides of the zone 3 are thermally insulated to improve the effective heat flow ratio further, the installation cost becomes lowest when the heat exchanger is of a regular square shape with a shortest peripheral length.