An automatic transmission comprising a transmission case and a brake (BR1, BR2), the brake (BR1, BR2) including: an inner stationary member coupled to the transmission case; an outer rotary member coupled to a given rotary member; a plurality of friction plates disposed between the inner stationary member and the outer rotary member; a piston configured to cause the plurality of friction plates to be engaged; and characterized by a hydraulic chamber for engagement, disposed radially inward of the inner stationary member, and to which hydraulic fluid is to be supplied, the piston being configured to be biased by the hydraulic fluid in an engaging direction to cause the plurality of friction plates to be engaged with each other, wherein the automatic transmission further comprises an oil channel forming member forming a supply oil channel for engagement through which the hydraulic fluid is to be supplied to the hydraulic chamber for engagement, wherein the piston includes: a pressing part provided at the outer circumferential side of the piston and configured to press the friction plates; a first hydraulic chamber forming part for engagement, provided at the inner circumferential side of the piston and forming the hydraulic chamber for engagement; and a first coupling part located at a radially intermediate part of the piston and extending radially so as to couple the pressing part to the first hydraulic chamber forming part for engagement, wherein the oil channel forming member includes: a bonding part located at a radially intermediate part of the oil channel forming part and coupled to one side of the inner stationary member in an axial direction; a second hydraulic chamber forming part for engagement provided at the inner circumferential side of the oil channel forming member, disposed at one side of the piston in the axial direction, and forming the hydraulic chamber for engagement; and a second coupling part located at a radially intermediate part in the oil channel forming member and extending radially so as to couple the bonding part to the second hydraulic chamber forming part for engagement, wherein the bonding part and the second coupling part of the oil channel forming member are disposed in the first coupling part. an inner stationary member coupled to the transmission case; an outer rotary member coupled to a given rotary member; a plurality of friction plates disposed between the inner stationary member and the outer rotary member; a piston configured to cause the plurality of friction plates to be engaged; and characterized by a hydraulic chamber for engagement, disposed radially inward of the inner stationary member, and to which hydraulic fluid is to be supplied, the piston being configured to be biased by the hydraulic fluid in an engaging direction to cause the plurality of friction plates to be engaged with each other, wherein the automatic transmission further comprises an oil channel forming member forming a supply oil channel for engagement through which the hydraulic fluid is to be supplied to the hydraulic chamber for engagement, wherein the piston includes: a pressing part provided at the outer circumferential side of the piston and configured to press the friction plates; a first hydraulic chamber forming part for engagement, provided at the inner circumferential side of the piston and forming the hydraulic chamber for engagement; and a first coupling part located at a radially intermediate part of the piston and extending radially so as to couple the pressing part to the first hydraulic chamber forming part for engagement, a pressing part provided at the outer circumferential side of the piston and configured to press the friction plates; a first hydraulic chamber forming part for engagement, provided at the inner circumferential side of the piston and forming the hydraulic chamber for engagement; and a first coupling part located at a radially intermediate part of the piston and extending radially so as to couple the pressing part to the first hydraulic chamber forming part for engagement, wherein the oil channel forming member includes: a bonding part located at a radially intermediate part of the oil channel forming part and coupled to one side of the inner stationary member in an axial direction; a second hydraulic chamber forming part for engagement provided at the inner circumferential side of the oil channel forming member, disposed at one side of the piston in the axial direction, and forming the hydraulic chamber for engagement; and a second coupling part located at a radially intermediate part in the oil channel forming member and extending radially so as to couple the bonding part to the second hydraulic chamber forming part for engagement, a bonding part located at a radially intermediate part of the oil channel forming part and coupled to one side of the inner stationary member in an axial direction; a second hydraulic chamber forming part for engagement provided at the inner circumferential side of the oil channel forming member, disposed at one side of the piston in the axial direction, and forming the hydraulic chamber for engagement; and a second coupling part located at a radially intermediate part in the oil channel forming member and extending radially so as to couple the bonding part to the second hydraulic chamber forming part for engagement, wherein the bonding part and the second coupling part of the oil channel forming member are disposed in the first coupling part.