PURPOSE: To perform tread adjustment steplossly keeping the wheel fitting condition as it is in such a way that a pair of hydraulic cylinders are provided on the axle housing, in the captioned equipment to adjust the distance between front wheels in a four-wheel drive tractor.

CONSTITUTION: In a structure that wheels 6 fitted to the ends of the sides of a flange shaft 11 which is pivotally supported at a final case 10 are turned through a differential gear 4, bevel gear trains 16, 17 and a final reduction gear 21 by means of revolution of a propeller shaft on the prime mover side, a cylinder 23 assembled by a cylindrical body 23a and a flange 23b is fitted to a flange 13a of the final case 10. And the cylindrical body 23a is inserted to be freely on the tubular part of an axle housing 1, the other end of a driving shaft 24 is inserted to be freely revolvable to the cylindrical body 23a. Besides, the outer cylinder 28 of a hydraulic cylinder 27 is provided integrally on the housing 1, and a built-in piston 30 is moved and controlled by oil pressure so as to perform tread adjustment.