To provide an underground water storage tank which can be installed for an individual dwelling house and is suitable for recycle of rainwater and utilization of cold energy, and by which a cost is reduced.

A crushed stone group 21 buried around a water storage tank 11 buried in a ground 3 reaches a gravel layer 9 in the ground 3. An infiltration window 23 is formed only at an upper part of the water storage tank 11 and is in contact with the crushed stone group 21. Rainwater 17 is supplied into the water storage tank 11 through a rainwater path 27. Heat of the rainwater 17 stored in the water storage tank 11 is utilized via a heat-pump pipe 29. The rainwater 17 stored in the water storage tank 11 is pumped up through a water-lift pipe and utilized as daily water.