A power transmission device includes a motor, a planetary gear mechanism having a portion that overlaps the motor in an axial direction, an axial direction wall part having a portion positioned between the planetary gear mechanism and the motor, a plate provided between the axial direction wall part and a pinion gear having the gear mechanism, and a box in which the motor, the planetary gear mechanism, the axial direction wall part, and the plate are housed. The box defines a breather hole that communicates with a breather chamber. The box has a radial direction wall part having a portion positioned between the plate and the breather chamber in a radial direction. The plate has a portion positioned between the pinion gear and the breather hole.