PURPOSE: To vary quantity of pre-load on bearing devices and nut member part so as to enable the pre-load on the both end parts to vary according to the moving condition of a movable table by varying the thickness of adjusting washers with current supply to piezoelectric elements.

CONSTITUTION: When a movable table 1 is quickly traversed, pre-loads on bearing devices 6, 6 and nut member 9 are almost removed, by supplying current respectively to adjusting washers 17, 17 inserted in the bearing devices 6, 6 on the both ends of a ball screw 5 and an adjusting washer 19 inserted in the nut member 9, so as to make these washers thin. Hereby the movable table 1 can be moved with a light load. On the other hand, at completion of positioning of the movable table 1 and at movement for precise interpolation, respective washers 17, 17, 19 are made to be thick so as to be given pre-loads to the bearing devices 6, 6 and the nut member 9. Hereby backlash in the driving system is eliminated, its rigidity becomes high, precise locus and positioning can be obtained, and adjusting thickness of respective adjusting washers can be performed at will even during movement of the movable table.