To derive the distribution of a moisture content (saturation rate) in the depth direction of a ground.

Heat is given to the ground 1 by a heating wire 4 and the amount of temperature rise corresponding to the depth is measured by an optical fiber cable 3 at a plurality of places in the depth direction separately. The saturation rate corresponding to the depth is determined on the basis of the measured amount of temperature rise and the distribution of the saturation rate in the depth direction of the ground 1 is derived therefrom.