To provide a method for determining the depth of an impervious wall to be constructed in the excavation of ground, in accordance with the actual measurement of the coefficients of the horizontal and vertical permeability of the in-situ ground.

The coefficients of horizontal and vertical permeability of the in-situ ground in a ground area to be excavated are each actually measured, and a predicted value of the amount of a decrease in surrounding water level with respect to the depth of the impervious wall and a predicted value of pumping discharge are computed according to the coefficients of the horizontal and vertical permeability, so that the depth of the impervious wall can be determined in such a manner as to satisfy the conditions that the predicted value of the amount of the decrease in the surrounding water level and the predicted value of the pumping discharge are equal to/smaller than target values, and that a construction cost, computed by the construction-cost consideration expression, is set to be the lowest one.