There are provided an axle housing, a rotational shaft provided to axially extend in a spindle of the axle housing, a wheel mounting cylinder which is mounted on an outer peripheral side of the spindle through wheel support bearings and to which wheels are mounted, and a planetary gear reduction mechanism for decelerating rotation of the rotational shaft, which is transmitted to the wheel mounting cylinder. A carrier forming a part of the planetary gear reduction mechanism is mounted in a non-rotating state on an open end side of the spindle. A cylindrical coupling member is provided between the spindle and the carrier, and provided with an axial one side outer periphery splined-coupled to an open end inner peripheral side of the spindle and an axial other side outer periphery spline-coupled to an inner peripheral side of the carrier.