The invention concerns an automatic gearbox, with a drive input shaft (3) which is connected to a shiftable reduction planetary gear set (10), with a drive output shaft (4) which is connected to a shiftable main planetary gear set (30), and with five shifting elements (A to E) by the selective closing of which six forward speeds can be engaged without range change. An output element of the reduction planetary gear set (10) can be connected via a first shifting element (A) to a first input element of the main planetary gear set (30) and via a second shifting element (B) to a second input element of the main planetary gear set (30). The drive input shaft (3) can be connected via a fifth shifting element (E) to a third input element of the main planetary gear set (30). A solar gear wheel (11) of the reduction planetary gear set (10) is in particular fixed on a cylindrical projection (43) of a gearbox housing wall (41). The reduction planetary gear set (10) is made as a Plus transmission with inner and outer planetary gear wheels (12, 15), whose webs (14, 17) are connected to one another.