Abstract: An air-breather device for an oil casing is filled with oil and accommodates a driving mechanism, e.g., a differential gear casing of a vehicle. The air-breather device comprises a cylindrical breather plug deflector attached to the wall of the oil casing and having an end opening into the oil casing, a breather plug having an open end opening into the breather plug in the oil casing and the other open end which opens to the outside of the oil casing, the other open end being normally closed by a spring-urged cover, and a baffle plate attached to the breather plug deflector on the upstream side thereof, as viewed in the direction of flow of the oil such as to intercept the flow of oil which tends to flow into the opening of the end of the breather plug deflector within the casing.