To estimate the three-dimensional moisture content distribution in the ground.

At least three boreholes 2A-2D are drilled into the ground 1, and at least three kinds of two-dimensional speed distribution data TAB-TCD of electromagnetic waves between each two boreholes are obtained by performing electromagnetic wave tomography. Then, the obtained two-dimensional speed distribution data TAC-TCD are converted into two-dimensional distribution data WAB-WCD of moisture contents by using a required transformation formula, and the moisture content in each grid of the three-dimensional grid model of the ground 1 is found by using the obtained two-dimensional distribution data WAB-WCD of moisture contents. Finally, the three-dimensional distribution of moisture contents in the ground 1 is estimated by using the found moisture contents.