To provide a water storage tank for water utilization and an installation structure of the water storage tank for water utilization capable of being utilized in such a state that its depth in the ground is shallower than a conventional one when water is used as a heat storage medium while applying ground heat as a heat source, and to provide a ground heat utilization-heating/cooling system of inexpensive installation work cost efficiently exchanging heat with the ground heat.

The ground surface-side contact face and the side face of the water storage tank 1 are covered with heat insulating layers 4, 5 to be utilized as the water storage tank 10 for water utilization, the water storage tank 10 for water utilization is installed in the ground in a state that the depth of the contact face at the ground surface side is within a range of 1-3 m, the water storage tank 10 for water utilization is made to serve as a heat storage tank, and the ground heat is utilized through the water storage tank 10 for water utilization to perform heating and cooling.