PURPOSE: To obtain the compact and highly accurate driving mechanism without necessitating heavy and rigid member, being connected so as to ride over guide rails, by a method wherein ball lead screws of a driving force transmitting mechansm for moving the work piece are provided at each guide rails so as to be positioned in the vicinity thereof.

CONSTITUTION: A main supporting table and a sub supporting table are provided at the left and right sides of the X-axis of a machine table 11. The guide rails 17, 19, parallel to the Y-axis of the mchine, are secured to the main supporting table while the ball lead screws 21, 23, constituting a part of the driving force transmitting device, are supported by bearing devices 25 and gear boxes 27 at the inside of the guide rails so as to be closed to the rails. The positioning and moving of the work piece in the X-axis direction are controlled numerically by a carriage 51 driven by a motor 55. The moving and positioning of the work piece in the Y-axis direction are effected by the driving rotation of a screw 21, driven by the output shaft 59 of the motor 57 through gears 61, 63 and the rotation of the screw 23, transmitted through engaging with the gears 75, 77 in the gear box 27.