Abstract: A floating liquid-storage tank has side walls of double-hull construction for storing a liquid lighter in specific gravity than water. The double-hull side walls consist of inner and outer hull sections and have watertight partition plates for dividing the hollows of the side walls into upper and lower ballast tanks. The outer hull sections of the upper ballast tanks are built to be resistant to the external water pressure and those of the lower ballast tanks are non-pressure-resistant. Holes through which the water outside can enter the lower ballast tanks are made in the outer hull sections of the lower ballast tanks. The partition plates are horizontally located at or slightly below the level of the intersection of the liquid-pressure distribution line and the external-water-pressure distribution line in the direction of depth of the storage tank in the full loaded condition.