An automatic transmission, wherein a first shaft is rotatably supported by the transmission case, and an input shaft is coupled with an engine output shaft. Additionally, a second shaft, comprising a long shaft and a short shaft rotatably mounted thereupon, wherein the short shaft has a helical input gear connected to an output gear of the first shaft by meshing engagement, a differential gear with a large helical drive gear connects to an output gear of the second shaft, and a speed-changing planetary gear connects to the short shaft.