PURPOSE: To achieve a lighter-weight and compact 5-speed automatic transmission (ATM) easily by adding two connecting means to a 4-speed ATM which is based on a light weight, a compact size and a low cost by itself.
CONSTITUTION: For a 5-speed ATM, when a transmission is made up with the first speed to the fourth speed, the fourth clutch C4 connects a carrier 11c' for a single planetary gear unit 11 and an output shaft 15' so as to be put into integral rotation. In such a state, actions are combined by controlling other connecting and locking means to form each of 4-speed transmission steps. During a 5-speed run the clutch C4 is disengaged and the fifth clutch C5 is engaged instead to connect sun gears 10s, 11s' in the unit 11 and the output shaft 15' for integral rotation. Also, the first clutch C1, the first brake B1 and the second one-way clutch F1 are engaged so that driving torque from an engine is conveyed to an input shaft 13 and input to a gear 10 in a dual planetary gear unit 10 via the clutches F1, C1.