[en] It is important to study the nature of underground deformations due to seismic forces in order to do the civil engineering of the ILC accelerator tunnel and detector hall. National Research Institute for Earth Science and Disaster Prevention (NIED) has installed a network of high sensitivity seismographs (Hi-net), consisting of 800 stations in a 20 km mesh spacing covering the Japan Islands. At each station, a tiltmeter is installed in a borehole at a depth of at least 100 m. We can derive the tilting effect on ground motion from the data. By using these Hi-net tilt data, we studied the underground deformation centered on the Kitakami candidate site during three month period before and after the Tohoku Earthquake (on the Pacific coast) in March 11, 2011. We have confirmed that the data were highly accurate in monitoring the normal earth tidal movements and we were able to study transient and permanent deformations due to the earthquake. (author)