An underground water storage tank for temporarily storing water overflowing from a gutter is provided. An underground water storage tank (20) for storing water overflowing from a gutter installed in a basement of a building or the like, wherein a first pipe (21) for guiding water in the gutter (40) to the underground water storage tank (20), Water is stored through an electric valve 24 that opens and closes one pipe 21, a water level sensor 25 that detects the level of water stored in the underground water tank 20, and a second pipe 22 that is connected to the middle of the first pipe 21. And a control unit 30 for controlling the electric valve 24 and the submersible pump 26. In the control by the control unit 30, the stored water level detected by the water level sensor 25 is a threshold value. The process which closes the electrically-driven valve 24 when exceeding is included. It can reduce the flooding of roads and inundation of houses during typhoons and heavy rains. [Selection] Figure 1
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