A power train of an automatic transmission realizing seven forward speeds and three reverse speeds includes an input shaft, a transmission case, a speed reduction unit including an output unit, a first clutch, a second clutch, a second brake, a third brake, and a fourth brake. The output unit generates seven forward speeds and three reverse speeds at the shift speed output member by operations of the first and second variable input members and third and fourth variably stopped members in response to operation of the clutches and brakes.