A traction nut has twist rollers pivotally supported so as to be rotatable in a state of defining an inclination angle with respect to an axis of a drive shaft, the inclination angle being equivalent to a lead angle; an open/close lever provided to switch between a friction-contact state and a disengaged state of the traction nut relative to the drive shaft; and a spring generating a biasing force such that the traction nut is brought into friction-contact with the drive shaft. In response to user operation, the open/close lever puts the traction nut in the disengaged state against the biasing force of the spring. When the user releases the open/close lever, the traction nut returns to the friction-contact state due to the biasing force of the spring.