To prevent the breakdown of a driving means and gears for driving an operation unit by providing a slip gear to be slipped in a direction opposite to the rotating direction when the force transmitted between components exceeds the predetermined threshold value.

In a gear train forming a neck part 4, a second neck part gear 27 is slipped by the constant external force so as to prevent the transmission of the input from outside to other gears. Namely, in the case where the forcible external force or the external force due to an impact is applied to the neck part 4, the external force is transmitted to the gear train inside of a gear box by a third neck part gear 28, but in the case where the external force more than the predetermined value is input, the second neck part gear 27 is slipped so as to prevent the transmission of the external force to the first neck part gear 26. Consequently, transmission of the external force to an electric motor 24 as a driving source can be prevented.