A power transmission device includes: a spindle case that includes a central hole through which an axle shaft penetrates in a central portion and forms a brake chamber in a region around the axle shaft at a base end portion; a wheel hub that is rotatably supported by an outer circumference of the spindle case and includes a final chamber, between the final chamber and the brake chamber, oil can flow, by communicating with the brake chamber through the central hole of the spindle case; a wet type brake mechanism that is contained in the brake chamber and operates so as to apply braking on rotation of the axle shaft with respect to the spindle case; and a deceleration mechanism that is contained in the final chamber and decelerates the rotation of the axle shaft to transmit the rotation to the wheel hub.