It has been desired to observe the three-dimensional displacements inside the model ground in laboratory tests in order to understand the mechanism of three-dimensional behavior of ground. However, as the existence of a sensor buried in model ground causes an influence to the strain in small size laboratory tests, no direct measurement of the strain inside the model ground in laboratory tests was conducted. In the present study, the displacements inside the model ground are obtained by measuring the intensity of the magnetic field induced by a small magnet buried in the model ground. By this newly developed device, it became possible to measure a radial strain in a model ground during the three-dimensional consolidation process.