19 and 20 are provided so as to enclose the outer periphery in such case one outer race 19 is large and is also the roller sleeve 12 in the ninth embodiment that is to say a dust preventing part 19b is provided to project on the side facing the shaft 11 in one end part of one outer race 19 and a rounded point 19a is provided to project in the bottom the small outer race 20 is fixed as a bearing race within the opening on the other end side the shape of this outer race 20 is substantially the same as in the ninth embodiment and is fixed to the inner peripheral wall of the opening of the outer race 19 the balls 15 are borne by the ball receiving groove 11a and two rounded points 19a and 20a the entry of dust and the like into the ball part is controlled by the dust preventing parts 19b and 20b on both sides to be minimum the thirteenth embodiment of the present invention shown in fig1 is a modification of the tenth embodiment a small diameter part 12a is formed on the inner periphery of the outer race 12 to step the inner periphery the dust preventing part 17b of the shaft 11 and the dust preventing part 16b of the inner race 16 are opposed to the stepped parts on both sides through dust preventing gaps the balls 15 are borne by the rounded point 17a on the shaft 11 side the rounded point 16a of the inner race 16 fixed to the shaft 11 and the ball receiving groove 12a of the outer race.