To downsize an automatic transmission in which an output gear is held in a cylindrical holding part installed on a sidewall.

The automatic transmission transforms a speed of a rotation of an input shaft 2 to which a power from a driving source ENG is transmitted into a plurality of stages, and freely outputs the rotation from an output gear 3. The transmission includes: the sidewall 1a which is run toward a radially inward from a transmission case 1; and the cylindrical holding part 1b which extends toward an axial direction from the sidewall 1a. The output gear 3 is pivotally supported to freely rotate along an outer peripheral surface of the cylindrical holding part 1b, and connection mechanisms B1 and F1 are arranged in the radially inward of the cylindrical holding part 1b.