PURPOSE: To find abnormal infiltration at an early stage thereof, by a method wherein the amounts of electricity between a plurality of electrodes arranged at different places on an earth retaining wall are measured, and on the basis of the change thereof, the places of water infiltration and the amount thereof are estimated.

CONSTITUTION: A plurality of electrodes 2 each are arranged at different places on an earth retaining wall 1 and the amount of electricity between the electrodes 2 is measured by a measuring apparatus 3. Impedance is desirable as the amount of electricity to be measured. After that, the amount of electricity measured is processed through a control means 8 and a change thereof is found. On the basis thereof, the place of water infiltration in the earth retaining wall 1 is estimated. Furthermore, the result of its measurement is indicated on an indicator 4 and on a printer 10, and the place of the water infiltration and the amount thereof are also indicated on the indicator 4. When the amount of the water infiltration reaches a specified value or more, abnormal infiltration is informed by a warning means 9.