Provided is a vehicle-use power transmission device to which is provided, within a housing (600) joined to the outer surface of an engine (200), an end cover (700) that shields an end part of an electric motor (311) on the engine (200) side, and the device is provided with an engagement mechanism (500) that is connected to an output shaft (201) of the engine (200) and that selectively stops the rotation of a rotating shaft (302) which is disposed coaxially with the electric motor (311). A sub-cover (710) through which the rotating shaft (302) passes is attached to a side surface of the end cover (700). According to the sub-cover (710), the end cover (700), and the rotating shaft (302), an accommodation chamber (620) is blocked with respect to the outside of the housing (600) in a liquid-tight state with an outer circumference smaller than the inner circumference of a stator (311S) of the electric motor (311), and the accommodation chamber is formed on the outer circumferential side of the rotating shaft (302). The engagement mechanism (500) is accommodated by the accommodation chamber (620).