To improve the maximum load ability of a friction body by eliminating the concentration of damage to a material, represented by equivalent stress or maximum shearing stress.

The change of a planetary roller 14 in the axial direction in a gap between a fixed wheel 12 and the planetary roller 14 and in a gap between the planetary roller 14 and a sun shaft 13 is so deterimed as to uniform damage to a material, represented by equivalent stress distribution or the maximum shearing stress distribution in the axial direction. In this way, the concentration of the equivalent stress or the maximum shearing stress at the edges of both axial ends of the planetary roller 14 is prevented. As a result, the static maximum load capacity and the dynamic maximum load capacity of the planetary roller 14 can be improved, equivalent to eliminating the concentration of damage to the material.