To provide a brake device of an automatic transmission which can efficiently ensure a space in a transmission case.

This brake device of the automatic transmission comprises a drum 32 of a drum clutch 30 connected to a sun gear 60s to constitute a Ravigneaux type planetary gear mechanism 60 disposed in a transmission case 100, a first brake which can lock the drum 32 to the transmission case 100 in a non-rotatable manner, and a second brake which can lock a one-way clutch C0 disposed in parallel to the first brake along the axis O1 and connected to the drum 32 to the transmission case 100 in a non-rotatable manner. The first brake is a hand brake B externally fitted to the drum 32, and the second brake is a multi-disk brake comprising a piston 75 to press a plurality of friction elements, and an actuator which is disposed in a cylinder 100r formed in the transmission case 100 to generate the pressing force in the piston 75.