PURPOSE: To simultaneously presume rear lateral pressure acting on an earth retaining wall efficiently at high degree of accuracy and coefficient of subgrade reaction.

CONSTITUTION: A first treatment means 42 fetches measuring data containing displacement of a wall, bending moment and axial force of a strut from the present analyzation section 2, designed value of rear lateral pressure as the measuring data, initial value of rear lateral pressure acting on the wall to be presumed and coefficient of subgrade reaction and covariance value of the error and, etc., as input data, and extended Karman filter treatment is repeated. A weighting means 44 is weighted with the error covariance value of estimate of the rear lateral pressure and coefficient of subgrade reaction obtained from the first treatment means 42. A second treatment means 45 uses the estimate obtained from the first treatment means 42 and weighted covariance value as the initial value, and the extended Karman filter treatment is repeated. A convergence decision means 46 decides the convergence of its estimate to obtain optimum estimate of the rear lateral pressure and coefficient of subgrade reaction.