PURPOSE: To facilitate the control of a pre-load and prolong the service life by aligning contact points between rolling elements lined in upper and lower rows respectively and raceway grooves of a track rail and casing, and central points of the rolling elements on a straight line, respectively, and crossing both the straight lines on the vertical center line of the rail and casing.

CONSTITUTION: Raceway grooves 9, 10, 11, 12 corresponding to raceway grooves 4, 5, 6, 7 of a track rail 2 are formed in a casing 8, where rolling elements 13, 14, 15, 16 are interposed under a predetermined pre-load. The rolling elements 13-16 can be endlessly circulated and rolled in the raceway grooves 4-7, 9-12, return holes 17-20 and direction changing paths. Contact points (a)-(f), (g)-(l) are aligned on straight lines L1, L2, and an intersection of the straight lines L1, L2 is crossed at an intersection 26 on a vertical center line L3 of the track rail 2 and casing 8. Therefore, an elastically deformed state due to the pre-load can be calculated accurately, thus facilitating management of the pre-load so as to maintain smooth sliding motion for a long period of time.