In this paper, torsional vibration characteristics of a seismically-isolated building constructed on an inclined bearing layer are investigated based on strong motion records and microtremor measurements. In order to elucidate the generation mechanism of torsional vibration, the torsional response of the building was reproduced using a 3D-frame model of the building and a 2D-FEM model of the subsurface structure. It is found that several types of torsional vibrations were generated in different frequency ranges. The warping torsional vibration, which was generated at 6.7 Hz, was found to be affected by the phase difference of the input motions.