To provide an underground heat storage device construction method, an underground heat storage device, and an underground heat storage method capable of improving heat storage efficiency by enhancing water retention of soil and improving the specific heat, even if the soil has low water retention and contains hard clayey objects, stones, etc.

The underground heat storage device includes a plurality of underground heat storage modules, each of which has a dugout formed in the soil, a hydrous layer in which the soil around the dugout is continuously sheared in the longitudinal direction and the sheared soil is made to contain water-absorbing resin, a heat exchanger arranged at a position where at least a part of the heat exchanger faces to the hydrous layer inside the dugout. On the upstream side of the flow direction of water in the soil, spaces between the hydrous layers of the adjacent underground heat storage modules become dense. On the downstream side of the flow direction of water in the soil, the spaces between the hydrous layers of the adjacent underground heat storage modules become non-dense.