Disclosed is an automatic of a vehicle comprising: a rotating shaft; a sun gear included at a circumference of the rotating shaft; a plurality of pinions engaged with the sun gear and rotated along an exterior circumference of the sun gear; a ring gear configured to be rotated as inscribed with an exterior circumference of the plurality of pinions; a case forming an exterior of the automatic transmission; and a supporting unit between the ring gear and an interior circumference of the case.