To increase the capacity of a lockup clutch without increasing an outside diameter and overall length of a hydlaulic transmission provided with frictional surfaces on both sides of a lockup clutch plate.

It is furnished with a torque converter cover 1 to rotationally driving a pump impeller 3 by rotating by rotation of an input shaft 2, a turbine runner connected to the side of an output shaft 5, a lockup piston 9 provided between the torque converter cover 1 and the turbine runner 4, divided into a lockup oil chamber A and a converter oil chamber B and free to nove in the axial direction of the output shaft 5 and a lockup clutch plate 10 provided in the lockup oil chamber A and free to move in the axial direction of the output shaft 5. A spline 1A is formed on an inner circumferential surface of the torque converter cover 1, a flange part 9c is provided on an outer peripheral part of the lockup piston 9, a spline 9A is formed on an outer peripheral part of this flange part 9c, and this spline 9c is engaged with the spline 1A on the inner circumferential surface of the torque converter cover 1.