Abstract: A system for detecting the position of a break in a water-barrier sheet placed in a waste disposal facility for isolating wastes from the ground of the waste disposal facility includes at least one optical fiber laid in a predetermined pattern between the ground of the waste disposal facility and the water-barrier sheet and constituting a temperature sensor capable of detecting a change in temperature caused when it is flooded with leakage water flowing out from a break in the water-barrier sheet at a temperature elevated under the effect of heat generated during biodegradation of the organic substances contained in the wastes, and a temperature measuring instrument connected to said optical fiber for detecting the position of the break in the water-barrier sheet via detection of the temperature change by said temperature sensor composed of said optical fiber.