PURPOSE: To prevent a locked state from being released when a heavy vehicle is loaded on a dump truck equipped with a vessel locking mechanism for transporting bulldozers, jumbos, etc.

CONSTITUTION: A dump truck equipped with a vessel driving mechanism for driving a vessel 2 from a horizontal state to an inclined erection state is equipped with a lock mechanism A for fixing the vessel 2 to a chassis 1 in the horizontal state, and the locking mechanism A is constituted of a vessel side locking member 22 and a chassis side locking member 20 which can be engaged with the vessel side locking member 22. Further, a transfer mechanism 20A for shifting the locking mechanism A to a locked state in the restoration oscillation of the vessel 2 to the horizontal state, lock holding structure RK for holding the locking mechanism A in lock state, and a release driving means UK for lock-release-operating the locking mechanism A are provided.