To improve maintenance capability by forming one side and the other side, which are in contact with, at least, a first or second hub of a torque transmission part, from an aramid based resin material.

Grooves are diametrically provided in flat faces 11A, 11B of a torque transmission part 11 in such a positional relation as crossing at right angles with each other and being opposed to a projection part 3A of a drive-side hub 3 and a projection part 4A of a driven-side hub 4, the projection parts 3A, 4A are fitted in the grooves, and the drive-side hub 3 and the driven-side hub 4 are connected to each other. An aramid based resin material having sufficient abrasion resistance and shock resistance and providing rigidity is used for the torque transmission part 11. This constitution can prevent friction caused by sliding between the projection parts 3A, 4A and the grooves, wear of a sliding face caused by microvibration of a drive source, and abrasion, hold rotation transmission accuracy for a long period without changing parts, and improve maintenance capability.