To provide an automatic transmission which can reduce a shock due to speed changing, while suppressing increase in an axial dimension.

An input shaft 11 is provided with a plurality of driving gears 21a-26a, and an output shaft 12 is provided with a plurality of driven gears 21b-26b for composing a plurality of gear trains by engaging with the driving gears 21a-26a. Power transmission gear trains among a plurality of gear trains are changed over by means of speed changing mechanisms 31-33, and driving torque in a running time is transmitted by selected gear trains. There is provided a by-pass clutch 35 for transmitting the driving torque, from the input shaft 11 to the output shaft 12 via the gear trains selected by a by-pass switching mechanism 36. When the speed is switched, the torque is transferred to the output shaft 12 due to the engagement of one by-pass clutch 35. Because this torque is made to differ according to the gear trains selected, the torque to be supplemented in the speed switching time is changed, according to speed changing conditions.