To conduct rational soil improvement for the support ground of a structure.

On the occasion of conducting soil improvement for the support ground of a structure, such improvement as to allow liquefaction of the ground after the improvement is conducted in order to reduce a seismic input to the structure in an earthquake exceeding a supposed scale. In this case, a judgment as to whether or not the liquefaction occurs in the earthquake of the supposed scale is conducted on the basis of the physical properties, e.g. an N value, of the ground after the improvement. The physical properties of the ground after the improvement are set so that the ground after the improvement be liquefied in the earthquake exceeding the supposed scale.