PURPOSE: To prevent insertion misses of turn supporting pins and facilitate to confirm dumping directions easy by providing detecting means to be operated by the insertion of freely attachable and detachable turn supporting pins to support a hinge mechanism in respective hinge mechanisms, and controlling a display means with output signals of the detecting means.

CONSTITUTION: In hinge mechanism A, B provided on both sides of the rear portion of a cargo box body 1 and hinge mechanisms C, D provided on both sides of the front portion of the cargo box body 1, a contact plane 5'a made by flattening a surface facing to a member 101 on the cargo box body 1 is provided on a fixing pin 5' screwed in a turn supporting pin 3, and on the other hand, an attaching hole 101a is provided in the member 101 facing to the contact plane 5'a, and a limit switch 11 is attached to the attaching hole 101a. And, for instance, in a backward dumping, when turn supporting pins 3 of the hinge mechanisms A, B are inserted and turn supporting pins 8 of the hinge mechanisms C, D are pulled out, the limit switch 11 can be operated by the fixing pin 5' on the turn supporting pin 3 to turn on a corresponding display lamp.