An automated manual transmission for a vehicle may include an input shaft configured to receive rotating force from a power source, an output shaft disposed parallel to the input shaft so that power is output from the output shaft, and a plurality of shift gear trains each comprising external gears respectively provided on the input shaft and the output shaft in such a way that the external gears circumscribe each other, the external gears forming a pair that embodies one of a series of shift ratios. Among the external gears that form a shift gear train in the plurality of the shift gear trains that embodies at least two adjacent shift ratios, either of two external gears that forms each of the shift gear trains may be connected to the input shaft or the output shaft by a planetary gear device.