PURPOSE: To easily form a turning groove by forming each half part of the turning groove onto a cap and a ball holder and forming a nearly U-shaped turning groove by assembling the ball holder and the cap.

CONSTITUTION: Arcuate grooves 21 and 22 connected with the edge parts of no-load guide grooves 14 and 15 are formed in the upper and lower stages onto a cap 12. Arcuate grooves 27 and 28 which are extended from the guide grooves 17 and 18 and connected with the edge parts of the grooves 21 and 22 are formed at the edge part of a ball holding device 11. The cap 12 is installed onto a ball slide part 9 by an installation screw 31. The first ball circulation passage 32 is formed from the first load guide groove 19, the first no-load guide grove 15, and the a pair of front and rear turning grooves 29 by installing the cap 12 together with a core 23 onto the installation part 10. At the same time, the second ball circulation passage 33 is formed from the second load guide groove 20, second no-load guide groove 16, and a turning groove 30.