To provide a method for detecting a position of water leakage occurrence suitable for inspection measurement of an impermeable sheet at completion of a disposal field.

Two layers, upper and lower, of impermeable sheets 16 and 12 are constructed in an original ground 10. A plurality of linear electrodes 18 are built in parallel above the upper impermeable sheet 16, and a plurality of linear electrodes 14 are built in an intermediate layer 15 below the upper impermeable sheet 16 intersecting with the linear electrodes 18 and in parallel to each other. One of the plurality of linear electrodes is selected in sequence for the upper and the lower side respectively, and power is applied between the selected upper linear electrode 18 and the intermediate layer electrode 14, and the electric current passing through at the time is detected to detect the presence or absence of the water leakage at the upper water impermeable 16. In particular, a liquid permeable protective layer 19 is provided above the upper impermeable sheet 16 so as to bury the upper linear electrodes 18, and the presence or absence of water leakage is detected by spreading liquid showing conductivity to the protective layer 19.