To avoid difficulty and a defect, accompanied by conventional arts, especially eminently reduce the occurrence of wear of a wear plate, prevent the generation of uneven rotation torque, and further develop and modify to optimize an operation condition for transmission of rotation torque as a whole.

In a coupling mechanism between the spindle head 1 of a universal joint spindle 4 and the roll shaft 2 of a roll, a wear plate 3 is charged under formation of shape integral properties between the spindle head 1 and a fit-in surface to transmit rotation torque of the roll shaft 2. A fastening device 20 is additionally attached to the spindle head 1, and under press operation on a wear plate by a fastening device and fastening of the spindle head 1 on the roll shaft 2, a shape integral coupling part for coupling with no clearance is formed.