PURPOSE: To obtain a water-sealing sheet which seals water by itself when damaged and follows the deformation of the natural ground, by a method wherein the water-sealing sheet with flexibility is formed by holding bentonite between two fibrous sheets with a water-sealing property and with a water- superabsorbing property.

CONSTITUTION: Bentonite 2 is held between fibrous sheets 1, 3 with a water- superabsorbing property, which absorb neutral water originating in ionic structure and expand extremely, and a water-sealing sheet with flexibility is formed. The water-sealing sheet is laid-down on natural ground 5 in a storage reservoir and soil covering 4 is done, if necessary. When the natural ground 5 is deformed, the water-sealing sheet is composed so as to follow its deformation, and when damaged, the sheet is composed so as to seal water by itself. In this way, the water can be surely sealed.