An automatic transmission realizes forward 10-speed and reverse 1-speed shift stages by including an input shaft; a first planetary gear set; a second planetary gear set connected through one part and selectively connected through another part with the first planetary gear set, and receiving power from the input shaft; a third planetary gear set connected with the first planetary gear set through one part and selectively connected with the first planetary gear set through another part, and selectively connected with the second planetary gear set; a fourth planetary gear set selectively connected to a transmission housing, selectively connected with the first planetary gear set, selectively connected with the second planetary gear set, and connected with the third planetary gear set through one part and selectively connected with the third planetary gear set through another part; and an output shaft connected with the third planetary gear set, and outputting speed-shifted power.