The authors propose a novel control method based on two observers to suppress the vibration of a flexible joint, even under fast motion. One observer is a disturbance observer and is used for the realization of an acceleration controller of the motor axis. The other observer estimates the link velocity and is effective for the suppression of torsional vibration. The disturbance observer estimates the total sum of the disturbance torques which are imposed on the motor axis. Feedback compensation by this estimated disturbance torque increases the servo stiffness and the robustness of the motor portion. The feedback of the estimated link velocity totally suppresses the torsional vibration of the flexible joint even under fast motion. The proposed control was implemented in a 16-b microprocessor with a digital signal processor for a laboratory test.