Natural soils are generally not homogeneous, though that is at rest. The thermal constants of soil varies as a function of depth. The temperature variations were measured at three points(Fig. 4) and the thermal properties of soil have been analyzed from the result of the measured data. The special discussion is focused on the thermal diffusivity. Judging from the result of those investigations, it is satisfactory to consider soils sonsisting two layers even if that is one layer.