In an automatic transmission (AT) in which an input shaft (Input) and an output gear (Output) are disposed on the same axis and three clutches (C1-C3) and two brakes (B1, B2) are selectively operated to provide six forward speeds, first and second planetary gear sets (GS1, GS2) are configured to consistently reduce the speed of input rotation and output the speed-reduced rotation, thereby enabling downsizing of them. The first and second planetary gear sets (GS1, GS2) are arranged in a transmission case at both the axial ends thereof. Third and fourth planetary gear sets (GS3, GS4) are connected to each other to have four rotation elements in combination, thereby constituting a Simpson planetary gear train. One of the four rotation elements (a fourth carrier (PC4) in an illustrated example) is connected to the output gear (Output). Rotations input and reduced in speed by the first and second planetary gear sets (GS1, GS2) are input to sun gears (S3, S4) of the third and fourth planetary gear sets (GS3, GS4), respectively. First to third clutches (C1-C3) and a first brake (B1) are arranged to surround the first and second planetary gear sets (GS1, GS2).