A transmission device comprising: a first clutch that includes a first clutch drum, a first piston, and a first engagement oil chamber; and a second clutch that includes a second clutch drum fixed to an outer circumferential portion of the first clutch drum, a second piston, and a second engagement oil chamber, wherein the first clutch drum includes a first outer cylinder portion that extends in an axial direction and is fitted with an outer circumferential portion of a first friction engagement plate, a first inner cylinder portion that extends in the axial direction inside the first outer cylinder portion and movably supports an inner circumferential portion of the first piston, and a first annular wall portion that is integrally formed with the first outer cylinder portion and the first inner cylinder portion so as to extend in a radial direction between the first outer cylinder portion (401 and the first inner cylinder portion; the second clutch drum includes a second outer cylinder portion that extends in the axial direction and is fitted with an outer circumferential portion of a second friction engagement plate, a second inner cylinder portion that extends in the axial direction inside the second outer cylinder portion and movably supports an inner circumferential portion of the second piston, and a second annular wall portion integrally formed with the second outer cylinder portion and the second inner cylinder portion so as to extend in the radial direction between the second outer cylinder portion and the second inner cylinder portion; characterized in that an inner circumferential surface of the second inner cylinder portion is joined with an outer circumferential surface of the first clutch drum so that the second annular wall portion axially overlaps the first annular wall portion of the first clutch drum as seen in the radial direction. a first clutch that includes a first clutch drum, a first piston, and a first engagement oil chamber; and a second clutch that includes a second clutch drum fixed to an outer circumferential portion of the first clutch drum, a second piston, and a second engagement oil chamber, wherein the first clutch drum includes a first outer cylinder portion that extends in an axial direction and is fitted with an outer circumferential portion of a first friction engagement plate, a first inner cylinder portion that extends in the axial direction inside the first outer cylinder portion and movably supports an inner circumferential portion of the first piston, and a first annular wall portion that is integrally formed with the first outer cylinder portion and the first inner cylinder portion so as to extend in a radial direction between the first outer cylinder portion (401 and the first inner cylinder portion; the second clutch drum includes a second outer cylinder portion that extends in the axial direction and is fitted with an outer circumferential portion of a second friction engagement plate, a second inner cylinder portion that extends in the axial direction inside the second outer cylinder portion and movably supports an inner circumferential portion of the second piston, and a second annular wall portion integrally formed with the second outer cylinder portion and the second inner cylinder portion so as to extend in the radial direction between the second outer cylinder portion and the second inner cylinder portion; characterized in that an inner circumferential surface of the second inner cylinder portion is joined with an outer circumferential surface of the first clutch drum so that the second annular wall portion axially overlaps the first annular wall portion of the first clutch drum as seen in the radial direction.