A power transmission device including a transmission, a case housing the transmission, a brake including a plurality of frictional engaging plates and a piston for pressing the frictional engaging plates and configured to non-rotatably hold any one of rotation elements of the transmission stationary to the case, and a support fixed to the case and including a ring-shaped wall portion extending in a radial direction of the transmission, a drum portion extending from the ring-shaped wall portion in an axial direction of the transmission and non-rotatably supporting a part of the frictional engaging plates, and a boss portion extending from the ring-shaped wall portion toward a side opposite to the frictional engaging plates in the axial direction and rotatably supporting another rotation element of the power transmission device.