A planetary gear device is provided with: a sun gear comprising external teeth; a plurality of planetary gears that comprise external teeth and that mesh with the sun gear; and a ring gear that comprises internal teeth and that meshes with the planetary gear. The ring gear is configured from at least one helical gear in which each of the internal teeth extends in a direction inclined with respect to the direction of the rotational axis of the ring gear. Each of the internal teeth of the ring gear comprises a first tooth surface in contact with the external teeth of the planetary gears and a second tooth surface on the opposite side from the first tooth surface. The pressure angle of the second tooth surface is larger than the pressure angle of the first tooth surface.
