PROBLEM TO BE SOLVED: To provide a work vehicle prime mover structure for preventing over-heated state of an engine caused by a poor work of a driving state changing-over means, for keeping a high suction capacity of a radiator fan and a high exhaust capacity of a dust exhaust fan and for showing a less number of times of repairing and inspection works.SOLUTION: The problem described above is resolved by a constitution in which when an operating state of a driving state changing-over means (45) is diagnosed poor by a control device (40) and a water temperature (T) of cooling water of an engine (E) is higher than a prescribed second set water temperature (T2) by the control device (40), a connected state of a threshing clutch (41A) for transmitting a rotating force power of the engine (E) to a threshing device (3) for threshing grain stalks is released to cause a rotation of the engine (E) to be reduced, and in turn the problem described above is resolved by a constitution in which when the operated state of the driving state changing-over means (45) is diagnosed poor by the control device (40) and the water temperature (T) of cooling water of the engine (E) is lower than the second set water temperature (T2) by the control device, the engine (E) is stopped.
