An automated manual transmission for a vehicle includes an input shaft and an output shaft disposed parallel to each other, a controller and a plurality of external gear trains, planetary gear devices and friction elements. Each external gear train comprises two external gears coaxially and respectively provided on the input and output shafts, and selectively engaged with each other to form a series of gear stages. Each planetary gear device selectively connects one of the two external gears in a corresponding external gear train such that a state of transmitting rotating force to a shaft, on which the selected external gear is provided, can be changed. Each friction element controls rotation of a rotational element of a corresponding planetary gear device. The controller controls the friction elements in accordance with driving conditions of the vehicle, thereby shifting the gear stages.