Abstract: In an automatic transmission having an air breather chamber for separating oil and air, entering the air breather chamber as oil bubbles, a shielding rib is located below an air-breather opening serving as an inlet-and-return port, for preventing oil, stored in a transmission casing assembly and splashed by rotary motion of a transmission rotating component part, from directly reaching the air-breather opening. An upside of the shielding rib is configured to be inclined downwardly with respect to a horizontal line in such a manner as to be oriented toward the transmission rotating component part, for receiving oil dripping through the air-breather opening with the upside of the shielding rib and for rapidly delivering the received oil toward the transmission rotating component part, using the inclined upside of the shielding rib.