Abstract: A method for detecting a leakage position in an impervious sheet, which comprises disposing a first and a second electrode units on an upper side and a lower side of the impervious sheet, respectively, the first electrode unit intersecting with the second electrode unit; short-circuiting all of electrodes in the first electrode unit; selecting a plurality of electrode sets one set after another set in the second electrode unit, each set comprising three electrodes adjacent to each other; supplying an AC voltage from an AC power source across a central one of each electrode set and the short circuited electrodes of the first electrode unit; measuring a potential difference between the other electrodes in the each electrode set to produce a measured voltage signal; and phase-detecting the measured voltage signal with a signal synchronized with the AC voltage from the AC power source to produce a detected signal, a series of the detected signals produced one after another changing in polarity due to a damage exi