In a power transmission device, a breather chamber having an opening on its one end side and extending coaxially with the countershaft is formed in a countershaft that transmits power from an output shaft of an automatic transmission to a differential mechanism. An inner end of a vent pipe placed so as to extend through a transmission case and that includes the inner end and an outer end each having an opening is inserted in the opening of the breather chamber. A plug, which closes a gap between an inner peripheral surface of the breather chamber and an outer peripheral surface of the inner end of the vent pipe so as to allow air to flow between the inside of the transmission case and the breather chamber and so as to restrict entrance of hydraulic oil into the breather chamber, is placed in the opening of the breather chamber.