PURPOSE: To improve the efficiency in the adjustment of position and to absorb the overload by comprising a first nut comprising a plurality of elements and made of a plastic material of good coefficient of friction and a second nut made of a mechanically strong material.

CONSTITUTION: A jack V comprises a first nut 9a made of a plastic material of good coefficient of friction and a second nut 9b having the mechanical strength. In the normal operation, that is, in a case where a position of a sheet structure 1 is adjusted by the jack V, the axial force applied to a screw 10 with the ordinary strength is absorbed by the nut 9a of low frictional force to improve the adjustment efficiency. On the other hand, in a case where the abnormal axial overload is acted on the jack V, the plastic nut 9a is distorted, and the clearance J1 or J2 of the nut 9b becomes lost, whereby the nut 9b absorbs the overload. As mentioned in the above, the efficiency in the adjustment of the position can be improved, and the mechanical strength can be maintained by absorbing the abnormal force.