Rolling element bores are provided in the outer circumferential portion of a disk-like main body. Each rolling element bore has an arcuate outer rim. Rollers each have a groove formed on the peripheral edge and extending along the entire circumference of the roller. The outer rim of each rolling element bore engages with the groove of the associated roller so that the roller is loosely fitted in the rolling element bore. Shafts are respectively formed on both sides of each roller. The shafts project from the center of the roller. An elastic ring is mounted on each shaft. Annular guide plates are respectively mounted on both sides of the disk-like main body so that the shafts of the rollers are supported by the guide plates.