PURPOSE: To get rid of all clearances in both radial and thrust directions between a leg member and a supporter utterly as well as to make improvements in rigidity and accuracy by forming a connecting part between a pair of isolatively installed supporters and plural leg members with a point-contact ball joint consisting of a pressurized ball.

CONSTITUTION: An output shaft of a servomotor 4 rotates a leg member 3, making a nut member 8 to be screwed in a pole screw 3a travel along this pole screw 3a. A movement of this nut member 8 is transmitted to a moving table 2 via an inner ring member, a steel ball 6c and an outer ring member 6a. Since movements between this pole screw 3a and the nut member 8 are simultaneously accired out by plural pieces of leg members 3, both up and down connecting parts 6 and 5 of the same number as the leg members 3 are tilted, so the moving table 2 performs the desired motion to the base 1, and thus a work on the base 1 is machined by a working tool of the moving table 2.