PURPOSE: To preferentially obtain high reliability while high accuracy and high rigidity are being enhanced by disposing each ball row in six lines at spaces equally parted from paired right and left focuses, and also symmetrically from right to left, and concurrently specifying the contact angle of each ball row at one side with respect to a water level.

CONSTITUTION: The unit includes a track base 20, a sliding member 30 which is freely slidable with respect to the aforesaid track base, and balls 40 in six lines which are interposed between the track base 20 and the sliding member 30. The track base 20 is made up of a track section 21 in an irregular octagon which is subjected to load applied to the sliding member 30 via the balls 40, and of a base section 22 which holds the aforesaid track base at a definite height over a flat installation surface. Ball rolling surfaces 23 are disposed so as to be formed over the track section 21 in such a way as to be equally parted from paired right and left focuses O1 and O2 while being symmetrically from right and left. And the contact angles γ of the respective ball rows with respect to a water level shall be roughly 0°, -γ, and 60°>γ>50° respectively. By this constitution, high accuracy and high rigidity can thereby be achieved, and static load rating can be made equal to dynamic load rating so that high reliability can thereby be obtained.