In a previous paper the authors developed a semiactive damper that generates electrical power, and carried out vibration tests using a 3-story benchmark structure at NCREE, Taiwan in 2006. At that time, the dampers were installed at 1st and 2nd floors. The damper has a large inertia mass by flywheel and controllable damping force by generator, and a load capacity of 30 kN. In the test, the damper at 1st floor was only controlled by Bang-bang control that was based on Lyapunov function. In this paper as the next step, in order to demonstrate more effectiveness of vibration suppression, the dampers are installed at all floors of the 3-story structure, and vibration tests using the same structure are carried out again in 2008. The control law which is proposed here is based on Lyapunov function or predictive switching control for all of damper at each floor. The structure has 9 m high, 3 m wide, 2 m span, 18 tons total weight. Several earthquake waves normalized to be 150 gal are inputted horizontally to the base by a shaking table, and the seismic responses of each floor are estimated quantitatively. It is obviously from the experimental results that the seismic reductions for acceleration and displacement in case of large inertia mass are much better than the case of small one, but control effect is decreasing. Because seismic reduction can be depended on a balance between inertial force and controllable damping force, and plus the one of the reason is time delay of the damper. At last, we can summarize that the semiactive damper is available for seismic isolation in practical use.