PURPOSE: To facilitate turning performance with switch-over automatically made between four wheel drive and two wheel drive by providing an automatic switch-over means which automatically engages/disengages power transmission to a front wheel power transmission shaft based on the ground speeds of rear wheels.

CONSTITUTION: A projected end 56 to which power is transmitted from the trailing end power transmission shaft of a change gear mechanism via an universal joint, a front wheel power transmission shaft and a universal joint 55, permits power to be transmitted to a driven body 37 via a spline shaft 30 of an outer package, the cam 39 of a drive body 41 and a slider 40 and both of clutch pawls 36, and also permits power to be furthermore transmitted to a right and a left front wheel via a front wheel input shaft 23. When the braking operation of one side rear wheel forces the ground speed of the other side rear wheel to be doubled at the time of turning a tractor, front wheels are pushed so as to be advanced, a gap is thereby made between the drive surface of the sliding cam 39 of the slider 40 and the drive surface of the sliding cam 39 of the drive body 41, this permits the slider 40 to be slid so as to come off from the clutch pawls 36 of the drive body 37 so that the front wheel are conditioned to no power transmission. After turning has been over, when the ground speed of the rear wheel is restored to normal ones, both of the clutch pawls 36 are meshed with each other so that the front wheels are conditioned to power transmission.