PURPOSE: To improve controllability by means of a controlling mechanism which can continuously shift the driving engine speed ratio for each differential gear, in a tractor provided those steering wheels coupled to and driven by individual differential gears, and other wheels incapable of steering located in both fron and rear.

CONSTITUTION: One pair of left and right wheels 5, 5 are controllably installed ahead of the body frame, and the both wheels 5, 5 are coupled together by means of a first differential gear 7. Besides, one pair of left and right wheels 8, 8 are controllably installed behind the body frame, and the both wheels 8, 8 are coupled together by means of a second differential gear 9. The first and second differential gears 7, 9 are interlocked and coupled with a third differential gear 10, and said gear 10 is coupled with the engine E. And, one pair of friction brakers 14, 14 are separately installed on an interlocking rotary shaft 12 of the both differential gears 7, 10 and on an interlocking rotary shaft 13 of the both differential gears 9, 10, and the desired purpose can be achieved by controlling each friction brake 14 through cylinders 15. Each cylinder 15 can be operated and controlled by controlling an electromagnetic control valve 16 through a controler 22.