PURPOSE: To prevent a ball screw from damage, when caused a fault like collision and the like, by breaking a shear substance and cutting a transmitting operation of rotational drive from the motor to the ball screw.

CONSTITUTION: Between an electric motor 2 and a ball screw 7, are interposed two couplings 3, 9 and in such manner with these two couplings 3, 9 likely to be bridged over, is easily removably provided a shear substance 10 in the peripheral recessed part of the coupling. When generated a fault like collision and the like, this shear substance 10 is broken along a rotational direction of the ball screw 7, causing a transmission of rotational drive from the motor 2 to the ball screw 7 to be disconnected, and a motion of the feed drive system is stopped. By a shearing of this shear substance 10 with the two couplings 3, 9 shifted relatively from each other in the rotational direction, a detecting substance 13 is rotated to a position to be detached from the coupling 9, with a limit switch 16 operated. Through an action of this switch 16 is detected a shearing of the shear substance 10, and the machine is all stopped in motion.