A power transmission device includes a pinion gear having a large pinion gear and a small pinion gear, a carrier that supports the pinion gear, a ring gear that engages with the small pinion gear, an oil supply unit positioned above a horizontal line that passes through a revolution center of the pinion gear, and a downstream side wall part facing a gear surface of the large pinion gear. The downstream side wall part is arranged to be adjacent to the oil supply unit further downstream in a revolution direction of the pinion gear than the oil supply unit when viewed from an axial direction.