Core disking and tunnel deformation like rockburst occurred at soft tuffaceous rock masses at a point 220 meters below the surface in Iwate Tunnel. The physical properties suggest that these rock masses have the possibility of swelling. The estimated earth pressure affecting the drilling hole is larger than five times the tensile strength of the rocks, which is considered to be a cause of core disking. Plate-shaped rock fragments continuously raveled like rockburst from the face of this tunnel, and deformation at the wall exhibited plasticity. The magnitude of these tunnel deformation is not influenced by either overburden or physical properties of rock masses, but mainly by geological structure.