PURPOSE: To reduce the manufacturing cost of a block by maintaining balls on races by means of a single lower cage.

CONSTITUTION: An inner circular groove 28 and an outer circular groove 29 are provided in the upper portion 26 of the base 27 of each single cage 25 secured to the lower end 16 of the wing 17 of a block 14, the role of the groove 28 being to complement a groove 18 in the block 14 for a set of lower balls 19, and the groove 29 complementing the groove 21 and constituting, together with a projection 30, a recirculation passage for the series of balls 19. Each set of balls 19, 19a has inclined, flat, or recessed horizontal side faces 34 and openings conforming to the contours of the series of balls 19a, 19a and adapted for allowing passage of part of each ball 19, 19a and for supporting them on the race 10 of the rail 1 are formed therein. Moreover, each set of balls 19, 19a is kept within the grooves 18, 23, 28 by a tongue piece 32 of a horizontal surface integrated with each terminal 33 of the single cage 25.