Wheel drive unit

A wheel drive unit (100) includes: a planetary gear mechanism (10) including an externally-toothed gear (24, 26) and an internally-toothed gear (28); a casing (30) integrated with the internally-toothed gear (28), a wheel (48) being attached to the casing, and the casing (30) transmitting rotation of the internally-toothed gear to the wheel; a bearing nut (56) configured to prevent axial movement of the casing (30); and a bearing (46) and an oil seal (70) as a detachment prevention member provided more toward an interior of a vehicle than the planetary gear mechanism (10) and axially fixed with respect the casing (30). An inner diameter of the detachment prevention member is smaller than an outer diameter of the externally-toothed gear (24, 26).