PURPOSE: To prevent the curvature deformation of a draw bar by fixing a support plate having a length extending over the upper surface of the second connecting plate in the longitudinal direction to the draw bar through bolts.

CONSTITUTION: A support plate 36 having a length extending over the upper surface of the second connecting plate 27 in the longitudinal direction is fixed to a draw bar 30 through bolts 34, 35, 37, 38 so as to nip the second connecting plate between the support plate and the draw bar, thereby permitting to partially absorb the bending force of the draw bar in the support plate, disperse the load and prevent the curvature of the draw bar, etc., without largely changing the whole design of the device.