To surely suppress rotational vibration with a small sum of investment with a comparatively simple method even if an existed object is to be altered by feeding back a rotational vibration suppression signal to a clutch.

The rotational output speed of a reduction gear 3 containing rotational vibration generated by the reduction gear 3 is measured by a sensor 5. An electric signal from the sensor 5 contains the displacement of the rotational output speed of the reduction gear 3, speed or acceleration and it is inputted to a controller 6. The phase of rotational vibration is inverted from the controller 6, the rotational vibration suppression signal (a) whose vibration is adjusted is fed back to the input of the clutch 2, the number of the rotation of the reduction gear 3 is increased/decreased and the rotational vibration of a load 4 is suppressed. The clutch 2 generates slip corresponding to the rotational vibration suppression signal (a) and gives rotation to the reduction gear 3 in a direction where the output rotational vibration of the reduction gear 3 is suppressed as rotational input.