An automatic transmission which can establish properly separated gear ratios of forward ten gears or eleven gears and reverse two gears, which is simple and compact and can comfortably change gears. A fourth sun gear of a speed-changing dual planetary gear is connected to a speed-reducing common ring gear of a speed-reducing dual planetary gear through a first clutch, a speed-changing common carrier is connected to an input shaft through a second clutch and is selectively fixed by a second brake. A third sun gear of the speed-changing dual planetary gear is connected to the speed-reducing common ring gear of the speed-reducing dual planetary gear through a third clutch, connected to the speed-changing common carrier through a fourth clutch, and selectively to a first sun gear through a fifth clutch, and is selectively fixed through a first brake. A speed-changing common ring gear is directly connected to an output shaft.