PURPOSE: To perform highly accurate positioning, with distortion of a bolt screw corrected, by independently providing rotary position detectors at both end sections of a ball screw and providing a servo controller averaging detected quantities from these detectors.

CONSTITUTION: A rotary position detector 3 is fitted to the output shaft of a servo motor 2, and the rotation of the servo motor 2 is conveyed to a ball screw 7 via gear devices 5, 6. The rotation of the ball screw 7 is conveyed to the other rotary position detector 12 via gear devices 10, 11. Detected quantities of the position detectors 3, 12 are averaged by a servo controller 1, and the average value is fed back. Accordingly, position detection can be performed with distortion of the ball screw corrected, and a highly accurate positioning device can be obtained with a simple structure at a low cost.