Abstract: An automatic transmission formed so that the axial displacement of a clutch drum is restricted without causing an increase in the manufacturing cost and number of man-hour. A first spline is formed on an inner circumferential surface of a carrier of first planetary gear mechanism, and a second spline meshed with the first spline is formed on an outer circumferential surface of a free end section of a hub portion of a clutch drum of a clutch. The second spline is formed by cutting process, and has a diagonally cut rising portion at a terminal end thereof. When the clutch drum is displaced from its set position toward an intermediate partition wall by a predetermined amount, an inclined surface of the cut rising portion of the second spline on the hub portion engages a radially inner corner edge portion of the carrier.