A power transmission device includes: a planetary gear mechanism including a sun gear that is an external tooth gear, a ring gear that is an internal tooth gear located coaxially with the sun gear, a pinion gear that is an external tooth gear and meshes with the sun gear and the ring gear, and a carrier that rotatably supports the pinion gear; a case member that accommodates the planetary gear mechanism; an extending member that extends from an outer peripheral portion of the ring gear toward one side in a thrust direction; and a holding member that is provided at an end of the extending member on a side opposite to the ring gear side in the thrust direction, the holding member being connected to the case member for holding the ring gear.