Active vibration control for mechanically flexible systems with slight damping is developed. It can be applied to active mass damper systems in tall buildings and active tendon systems in flexible spacecraft. This paper presents a study of active control (damping) of structural vibration using an active mass damper system consisting of a force actuator, a sensor, and a feedback controller. Active damping of a multi-derree-of-freedom system is attempted by employing a single force actuator. The effectiveness of this active mass damper system and the fundamental problems in practical application are shown experimentally by a model of a 4-storied structure.