PURPOSE: To facilitate mounting of an attachment and to prevent missing of a constituting part.

CONSTITUTION: A pair of brackets 1 each having a pivotal support shaft 2 matching and protruding from the inner surface thereof is fixed in a stepped recessed part 80a formed in the back of a attachment 80. A lock plate 4 is vertically movably suspended from each of the pivotal support shafts 2 and a hook main body 5 is rotatably connected and supported thereto through a pair of links 3. When a lower hook 82 is locked to a lower fork bar, by rotation of the lower hook around the pivotal support shaft 2 through the links 3, the hook main body 5 positioned below the lock plate 4 situated in an upward movement position is displaced obliquely upward to engage it with the lower fork bar. The lock plate 4 intervenes downward in a gap formed through the engagement and oblique downward displacement of the hook main body 5 in an engaged state is prevented.