The chatter suppression described in this paper is based on the application of active dampers to a slender boring bar. Chatter vibration signals detected by a pickup are fed to a computer. After calculating the chatter frequency and the corresponding phase shift parameter, the computer supplies the amplified signals to piezoelectric actuators with the same phase as that of the vibration velocity of the boring bar. As a result of this, the actuators generate damping forces ; that is, they act as active dampers. It has been confirmed in cutting tests that the active damping system adapts to change and fluctuation of the chatter frequency and suppresses chatter well. Furthermore, it has become clear that there is an optimum position of the active damper for chatter suppression.