An automatic transmission comprising a torque converter and a change speed transmission having a plurality of planetary gearsets and friction elements is provided with an input shaft supported by a transmission case. On the input shaft is mounted a small-diameter first drum. In the first drum are mounted a first clutch having a clutch piston, friction plates, separate plates and a clutch hub. On the input shaft is further mounted a large diameter second drum, in which are disposed a second clutch having a clutch piston, friction plates, separate plates and a clutch hub.The friction plates and separate plates in the second drum are disposed radially outside on top of the friction plates and separate plates in the first drum.