An underground water storage tank which rarely causes water leakage, and which largely increases an amount of water to be stored therein having an internal water storage sub-tank 11 provided by coating an assembly of water-bearing materials 12 obtained by combining multiple water-bearing materials 12 with one another with a first water-impermeable sheet 14; soil pressure absorption plates 26 provided outside the internal water storage sub-tank 11; and a second water-impermeable sheet 27 for coating the soil pressure absorption plates 26. The water-bearing materials are rigid pipes 12 made of resin and the assembly of the water-bearing materials is formed by mutually and parallel coupling the multiple rigid pipes 12 by pipe coupling means 13 in a state that the multiple rigid pipes 12 are vertically set and arranged at predetermined intervals therebetween in a horizontal direction.