PURPOSE: To speedily and stably control the object to be controlled by providing an observer which estimates generalized disturbance and feeding the estimated value of the observer back to a driving part.

CONSTITUTION: Disturbance torque τm operates on the input torque 'm of the driving part 2. Then a speed conversion part 4 converts the angle θm of rotation of the driving part 2 into θm/N. In this case, N may be larger than 0, but N is set normally larger than and equal to 1 and rotation is reduced in speed and outputted. Further, disturbance torque τdo operates on the rotation of the controlled body 1. For the purpose, the observer 6 estimates the generalized disturbance TXdmo converted assuming that the total of the disturbance τdm, the actual torque such as disturbance torque τdo, and disturbance torque including equivalent torque generated owing to parameter variation, etc., operates on the driving part, and feeds it back to the input side of the driving part 2, thereby performing control so that desired characteristics from input torque τm* to the control system to θ0 (angle of rotation detected by a detection part 5 for output rotation angle) are obtained.