A wheel drive unit includes: a planetary gear mechanism including a planetary gear and an internally-toothed gear; a casing integrated with the internally-toothed gear, a wheel being attached to the casing, and the casing transmitting rotation of the internally-toothed gear to the wheel; a bearing nut configured to prevent axial movement of the casing; and an oil seal provided more toward an interior of a vehicle than the planetary gear mechanism and axially fixed with respect the casing. An inner diameter of the detachment prevention member is smaller than an outer diameter of the externally-toothed gear.