PURPOSE: To facilitate size reduction while attaining multistage in an automatic transmission for transverse engine-mounted vehicle.

CONSTITUTION: This automatic transmission is provided with a driving gear 21 disposed, in connection with an output element R2 of a main shifting mechanism 1, on a main shifting shaft 10 formed with the main shifting mechanism 1, a driven gear 22 fitted on a differential gear shaft 40, a reduction mechanism 2 of chain transmission by a chain 23 wound around the driving gear 21 and the driven gear 22, and a sub shifting mechanism 3 connected to the differential gear 4 in the diametrical inner circumference side of the driven gear 22 enlarged by the reduction mechanism. It is possible to decrease axial dimension by overlapping the driven gear 22 and the sub shifting mechanism 3 in the diametrical direction.