PURPOSE: To recline a sheet in a cabin by openably coupling with a hinge part wall of the front wall of a dump body for blocking the opening formed at the lower part of a back panel of the cabin.

CONSTITUTION: A cabin 4 and a dump body 6 are respectively independently mounted on a frame. An opening 12 is provided at the lower part of a rear window 2 of a back panel 10 of a cabin 4. On the other hand, a back lid panel 18 mounted on a dump body floor 15 is openably provided with hinge 14 on the front side of the body 6. A seal rubber 28 is provided over the overall periphery at the opening 12. When the panel 18 is erected to block the opening 12, the panel 18 is contacted with the rubber 28. When the panel 18 is disposed horizontally, the opening 12 is opened, and the cabin 4 communicates with the body 6. The panel 18 is held in erected state by a locking unit 30.