PURPOSE: To eliminate backlash between a case and a link plate at assembly and reduce the number of parts and assembling manhour.

CONSTITUTION: The rigid body part 6 and the engaging pieces 7 extended from a link plate 5 are inserted into the boss part 2 provided on a case 1, the rigid body part 6 is slidably contacted with the inner circumferential face 4 of the boss part 2 so as to rotationally move, and the taper faces 9 of the engaging claws 8 provided on the extreme ends of the engaging pieces 7 are engaged with the opening end part of the boss part 2. Hereby, slip-out of the link plate 5 from the case 1 is prevented.