The present invention provides an automatic transmission having a main bearing disposed between the inner surface of a boss portion of a first drum and the outer surface of a cylindrical support, for rotatably supporting the first drum. An auxiliary bearing is press-fit into one end of the interior of the boss portion of the first drum and surrounds the cylindrical support with an annular gap therebetween. The auxiliary bearing contacts the cylindrical support only when the first drum tilts relative to the central axis of the transmission, and has a shorter axial length than that of the main bearing.