When a screw equipped with a check ring is moved forward, torque acting on the screw is detected, and whether the check ring is closed is determined on the basis of the detected torque. The screw is rotated in reverse when the check ring is determined not to be closed, and the reverse rotation of the screw is stopped when the check ring is determined to be closed. The reverse rotation of the screw is thereby controlled so that the screw is rotated in reverse by an optimal amount for reliably closing the check ring.