The inventive automatic transmission comprises first, second and third gear sets 10, 20, 30 each formed as a single-pinion type planetary gear set, first and second clutches 40, 50, and first, second and third brakes 60, 70, 80. A first sun gear 11, a first ring gear 14, a second ring gear 24, a third sun gear 31 and a first carrier 13 are continuously interconnected with a second sun gear 21, a second carrier 23, a third carrier 33, an input shaft 4 and an output gear 7, respectively. The first clutch 40 is operable to selectively connect and disconnect the first sun gear 11 and the second sun gear 21 to/from the input shaft 4, and the second clutch 50 is operable to selectively connect and disconnect the first ring gear 14 and the second carrier 13 to/from the input shaft 4. The first brake 60 is operable to selectively connect and disconnect the first ring gear 14 and the second carrier 23 to/from a casing 6. The second brake 70 is operable to selectively connect and disconnect the second ring gear 24 and the third carrier 33 to/from the casing 6, and the third brake 80 is operable to selectively connect and disconnect the third ring gear 34 to/from the casing 6. The clutches and brakes are selectively placed in their engaged states to establish six forward speed stages and a reverse speed stage.