It has been reported the vibration generated by the vertical motion of a large number of audiences in a live hall during a live performance brings unpleasant vibration to surrounding buildings. As a countermeasure against this vibration problem, we have developed a vibrationproof floor system using inertial mass device and introduced the system to an actual live hall. Excitation tests in principle model and the actual hall were conducted to demonstrate the effect of the system. As a result of the tests and simulation analysis, it is proved the proposed system is very effective on the vibration problem.