To increase the contact surface of a ball and an outer wheel so as to ensure sufficient strength by partitioning a space between the outer wheel and an inner wheel by first annular projection and a second annular projection, arranging the ball in annular three rows, and being brought into contact each ball with first and second guide surface and the inner wall of an auxiliary wheel.

A plurality of balls are housed in a space 3 formed between first and second guide surfaces 12a, 22a. First and second annular projections are bounded by those balls 4, are separated by the annular projection, and are arranged in an annular double rows. In the ball 4 of a left (right) side, the right (left) side upper surface thereof is brought in contact with the left (right) side guide surface 12a of the first annular projection 12, and the right (left) side lower surface is brought in contact with the left (right) side guide surface 22a of the second annular projection 22. In the ball 4, the left (right) side central part is brought in contact with the inner wall of the auxiliary wheel 5 of the left (right) side in a point shape. Each ball 4 is brought in contact with the outer wheel 1 and the inner wheel 2 at two points of the upper and lower surfaces in a surface shape.