The invention provides an active mass damper by use of which a machining apparatus can be downsized. The active mass damper includes a support unit for supporting a weight movably and horizontally, a motor for driving the weight, and a controller for controlling the motor. A torque command signal to be given to a motor of an X-table and a torque command signal to be given to a motor of a Z-table are put into the controller of the active mass damper so that the motor for driving the weight can be driven by feed-forward control using the torque command signals and feedback control based on the displacement of the weight.