It is possible to prevent generation of failure in a dump truck and to diagnose a true cause of failure in a short period of time. Thus the present failure diagnosis method diagnoses an operation and function of at least one of respective devices, such as an engine at a predetermined engine rotational speed, a transmission, respective axles, respective suspensions, hydro-pneumatic systems, and a brake while a vessel body (4) descends after evacuation of a load. The failure diagnosis device comprises body operating means (50) for raising or lowering a vessel body, a direction switching valve (7) for receiving a command from the body operating means to lower the vessel body, and a self-diagnosis control device (11) for using state detecting means (20) to detect and store a state of at least one of the respective devices at a predetermined engine rotational speed when the body operating means is at a predetermined operative position and for comparing a magnitude of a detected value and a rate of change with reference values on the basis of hysteresis data when carrying out failure diagnosis.