PURPOSE: To accumulate heat in underground inexpensively and efficiently by a method wherein the particles of a solid heat accumulating agent integrated with the ground and a bag body of a synthetic resin sheet having flexibility are burried into the ground.

CONSTITUTION: A layer structure consisting of the constitution of particles in which the adiabatic property of a layer consisting of gravel, sand, earth and the like and a heat accumulating property due to heat transmission are improved is made. According to this method, a layer having a high heat accumulating property is formed near a heat accumulating layer while the adiabatic layer containing a plenty of air, in which the natural convection is not effected, and having a high adiabatic property is formed at the outside thereof. According to these layer constitution, heat may be supplemented from the adiabatic and heat accumulating layer in accordance with the heat emission from the heat accumulating layer while the heat may be accumulated or emited in accordance with seasons, furthermore, earth or aggregate, existing in the neighborhood may be utilized inexpencively.