PURPOSE: To suppress slips and prevent wheels from devastating a field or being sunk by reducing the number of revolutions of driving wheels automatically when a certain amount of slips occurs.

CONSTITUTION: Detection results by a revolution speed detection unit 1 for front wheels and a revolution speed detection unit 2 for rear wheels are fed into a comparator 5 through arithmetic units 3, 4 respectively. The comparator 5 is a unit to determine the magnitude of slips by comparing the revolutions of front and rear wheels, and the obtained results are fed into a decision unit 6, which is a unit to decide whether to operate a deceleration unit 7 or not based on the magnitude of slips obtained by the comparator 5, thus the decision unit 6 operates the deceleration unit 7 when the magnitude of slips has reached a preset value or above and stops the operation of the deceleration unit 7 when the amount of slips has been reduced below the preset value.