PURPOSE: To invariably keep the stored water in a water storage tank at the full state by forming the water storage tank with a cylindrical wall body, a bottom plate, a top plate, catchment holes provided through the top plate, and a screen material fitted at the upper section of the top plate.

CONSTITUTION: This water storage tank is formed with a cylindrical wall body 1, a bottom plate 3, a top plate 7, multiple catchment holes 13 provided through the top plate 7, an entry hole 11 communicated with the ground surface through the top plate 7, and a screen material 8 fitted on the whole upper face of the top plate 7. The screen material 8 is designed to be necessarily located below the ground water level. The ground water is guided from slits of the screen material 8, and it flows into the water storage tank through the catchment holes 13 to fill the water storage tank to the full. When the stored water 17 in the water storage tank is used at the ordinary time, ground water is naturally replenished into the water storage tank invariably. When the stored water 17 is to be used as fire extinguishing water, an entry hole cover 12 is opened, a drainage pump is set to a catchment groove 4, and the stored water 17 is sent to the ground surface.