A positioning arm (1) comprises in its interior two continuous extension shafts (2) at the ends of which guide rollers (3) are mounted which accommodate a toothed belt (6) each. Said toothed belt is guided across deflection rollers (4) in the end sections of the trajectory of the positioning arm (1). At its free ends the toothed belt is fixated on a chassis (7) of the means. The inventive means effect a pulley-type reduction of the motive forces while providing an increased positioning precision.