The purpose of this study is to clarify the seismic performance of piled raft foundations with ground improvement based on seismic observation records. The monitored building, which is a twelve-story base-isolated building, is located on loose silty sand underlain by soft cohesive soil in Tokyo, Japan. On March 11, 2011, the 2011 off the Pacific coast of Tohoku Earthquake struck the building site, the seismic response of the soil-foundation-structure system was successfully recorded during the earthquake. Axial force and bending moment of two piles, earth pressure and pore-water pressure beneath the raft, and accelerations of the ground and structure were measured. It was found that the peak acceleration reduction of input motion caused by ground improvement. The sectional forces of piles at the pile head and at intermediate depth were almost same behavior. The bending moments increased during large ground deformation.