PURPOSE: To enable simple and secure adjustment of the front wheel tread width by making a pinion and a rack, installed between the extension tube and the fixed case of a front wheel drive unit, and expanding/contracting said tube.

CONSTITUTION: The central section of a fixed case 5 is formed into a large diameter differential gear case 5a within which an extension tube 14 is fitted with a free sliding movement. A rack 14a is formed at one side of the external periphery of said tube 14 to engage with a pinion 9, whose pinion shaft 8 sticks out of the external surface of the fixed case 5 and the protrusion is formed into a spanner grip. Accordingly, a change in the front wheel tread width can be performed by rotating the spanner grip either to expand or contract the tube 14. After the expansion/contraction operation, a lock pin 7 is to be rotated to engage and lock a pin hole 14a of the tube 14, restricting unnecessary sliding movement of the tube 14.