A driving force transmission device includes a sun gear that rotates upon receipt of a driving force, an internal gear that has a hollow portion and internal teeth on an inner surface of the hollow portion, the internal gear being disposed coaxially with the sun gear, a planetary gear that rotates and revolves in mesh with the sun gear and the internal gear, a rotating member that supports a rotating shaft of the planetary gear, and rotates about an output shaft as the planetary gear revolves, and a retaining member that contacts with one end of the rotating shaft of the planetary gear on one surface side of the rotating member to retain the rotating shaft.