PURPOSE: To prevent any displacement of a retainer and deterioration of durability by retaining, in pockets disposed in the retainer, a steel roller and an elastic roller made of an elastic material into which fluid is sealed, where a diameter of the elastic roller is made slightly larger than that of the steel roller.

CONSTITUTION: A pocket 3a for a steel roller and a pocket 3b for an elastic roller are disposed in a retainer 1. A steel roller 4a is retained in the pocket 3a for the steel roller; and an elastic roller 4b, in the pocket 3b for the elastic roller. In the elastic roller 4b, incompressible fluid 5 such as silicone oil is sealed inside of hard rubber or the like, and an outer diameter is lightly larger than that of the steel roller 4a. Consequently, contact between a raceway and the elastic roller 4b is highly close without generating any sliding therebetween. Therefore, no displacement of the retainer due to oscillating motion can dispense with a displacement preventing mechanism, thereby preventing deterioration of durability despite of light-weight and a small size.