Rotational motion of a male screw member is converted to linear motion of a female screw member that is screw engaged with the male screw member, thereby causing movement of a rod member for clamping an object. When a load from the object becomes larger than a predetermined value, and exceeds the frictional force of a viscous material and a friction plate, the female screw member is gradually turned and clamping continues while movement of the rod member is decelerated.