60 39 formed integrally with a sun gear 11a 39 of the rear planetary gearset 11 39 through a one way clutch owc4 39 a friction ring member k4b 39 is disposed within the cylindrical member 55 39 the cylindrical member 55 39 and a piston k4c 39 disposed in front of the cylindrical member 55 39 form therebetween a hydraulic pressure chamber k4d 39 a clutch k5 39 is connected by a spline k5a 39 to the cylindrical hollow shaft 60 39 formed integrally with a sun gear 11a 39 of the rear planetary gearset 11 39 and has a friction ring member k5b 39 installed within the cylindrical member 55 39 the clutch k5 39 further has a piston k5c 39 disposed in front of the piston k4c 39 of the piston k4 39 so as to form a hydraulic pressure chamber k5d 39 a return spring k5e 39 is disposed in front of the piston k5c 39 for biasing both the clutches k4 39 and k5 39 the transmission casing 17 is integrally formed at its mid portion with a partition wall 17a which extends radially inward between the counter gear 21 and the rear planetary gearset 11 39 and holds a thrust bearing 17b the partition wall 17a with the thrust bearing 17b makes the automatic transmission very rigid in both the radial and axial directions as is clear from the above in order to change the four forward speed automatic transmission at4 39 into the five forward speed automatic transmission at5 39 it is enough to attach the clutches k4 39 and k5 39 and the one way clutch owc4 39 as friction elements of the fifth speed gear unit x 39 after.