In oscillating an oscillating body, a control unit obtains a load torque due to the gravity acting on a drive motor at least at one angular position defined about an axis of rotation. The control unit calculates a specified maximum acceleration depending upon if the load torque Q is acting in a direction in which it hinders the acceleration or deceleration of the drive motor 15 or is acting in a direction in which it assists the acceleration or deceleration. An acceleration for the oscillating body is set so as not to be greater than the calculated specified maximum acceleration. The control unit adjusts the acceleration of the oscillating body depending upon the load torque of when the oscillating body 13 is accelerating or decelerating.