Excitation tests were carried out for an experimental model of a mass damper with a 200 kg moving mass which was installed on the roof of a 5-story building model with a total mass of 6029 kg and a height of 3.58 m. The 1st mode (2.4 Hz) and 2nd mode (7.3 Hz) of the building vibration were controlled, using a variable feedback gain control law to keep the mass damper displacement within a limit. Under the control law, the mass damper can work against not only winds and weak earthquakes but also strong earthquakes. Two types of variable gain control law were tested, one of which used a crisp switching rule of the gain and the other a fuzzy switching rule. The test results demonstrated the effectiveness of the control laws. Furthermore, the influence of the reduction in the number of sensors and robustness of the control system, including the modal filter, were tested with satisfactory results.