An axle driving unit having a housing (21,22) for supporting a first axle (17) for mounting thereon a first driving wheel (3L), and a longer second axle (18) for mounting thereon a second driving wheel (3R). The axle driving unit is eccentrically mounted on a body frame (1) in proximity to the first driving wheel (3L). A speed change transmission (T) is disposed in an enlarged region of the housing that extends substantially perpendicularly with respect to the axles (17,18). The axle driving unit can thus be easily disposed to the side of a chute (6) of a rear-discharge type lawn tractor.