An automatic transmission comprising: an output gear ; a radial bearing ; a transmission case that includes a cylindrical portion ; a cylindrical boss portion configured to be supported by the cylindrical portion through the radial bearing to rotate, the cylindrical boss portion being configured to protrude from the output gear to a direction in which a thrust force of the output gear is produced; a friction engaging element disposed such that the output gear is located between the radial bearing and the friction engaging element ; a piston configured to apply a pressure that causes the friction engaging element to be engaged, the piston being disposed on an outer diameter side of the cylindrical portion of the transmission case , and the piston being disposed to slide in an axial direction of the transmission case ; a first retaining ring disposed between a first outer circumferential groove that is provided in an outer circumferential surface of the radial bearing , and an inner circumferential groove that is provided in an inner circumferential surface of the cylindrical portion of the transmission case ; a ring piece provided to an outer ring of the radial bearing , the ring piece being configured to protrude outward in a radial direction, and the ring piece being in contact with an output gear side end surface of the cylindrical portion of the transmission case in the axial direction; a second retaining ring configured to lock in a second outer circumferential groove that is provided in an outer circumferential surface of the ring piece ; and a return spring disposed at an outer circumference of the ring piece , the return spring being configured to urge the piston in a direction opposite to a direction in which the pressure is applied by the piston when the piston stops applying the pressure that causes the friction engaging element to be engaged, and the return spring being configured to retain in the axial direction by the second retaining ring . an output gear ; a radial bearing ; a transmission case that includes a cylindrical portion ; a cylindrical boss portion configured to be supported by the cylindrical portion through the radial bearing to rotate, the cylindrical boss portion being configured to protrude from the output gear to a direction in which a thrust force of the output gear is produced; a friction engaging element disposed such that the output gear is located between the radial bearing and the friction engaging element ; a piston configured to apply a pressure that causes the friction engaging element to be engaged, the piston being disposed on an outer diameter side of the cylindrical portion of the transmission case , and the piston being disposed to slide in an axial direction of the transmission case ; a first retaining ring disposed between a first outer circumferential groove that is provided in an outer circumferential surface of the radial bearing , and an inner circumferential groove that is provided in an inner circumferential surface of the cylindrical portion of the transmission case ; a ring piece provided to an outer ring of the radial bearing , the ring piece being configured to protrude outward in a radial direction, and the ring piece being in contact with an output gear side end surface of the cylindrical portion of the transmission case in the axial direction; a second retaining ring configured to lock in a second outer circumferential groove that is provided in an outer circumferential surface of the ring piece ; and a return spring disposed at an outer circumference of the ring piece , the return spring being configured to urge the piston in a direction opposite to a direction in which the pressure is applied by the piston when the piston stops applying the pressure that causes the friction engaging element to be engaged, and the return spring being configured to retain in the axial direction by the second retaining ring . <EOS>