PURPOSE: To prevent the deformation and damage of a lead screw by forming a stop means for releasing the engagement between the screw of a moving body and the spiral screw of the lead screw, at the moving stroke terminal of the moving body.

CONSTITUTION: An unthreaded part is provided at one end of a lead screw 29b to form a non-driving shaft part 29b1. The outer diameter of this non-driving shaft part 29b1 is set smaller than the inner diameter of the screw hole 29a3 of a driving block 29a so as to make the driving block 29a freely movable in the non-driving shaft part 29b1. In the non-driving shaft 29b1, a plate spring 32 having elastic force in the other end direction of the lead screw 29b is disposed being fixed to a rail part 28 so as to energize the driving block 29a brought into contact with the plate spring 32 in the return direction. The driving block 29a reaching the non-driving shaft part 29b1 is therefore released from the screwed state with the lead screw 29b and stops receiving the energizing force of the plate spring 32. The lead screw 29b is thereby rotated without receiving torsional force.