A driving force transmitting device comprising: an output shaft which can output a rotating force, and a casing which supports the output shaft so that a leading tip part of the output shaft can expose the outside, wherein the output shaft is provided on the casing so that the tip end surface thereof is flash with or substantially flash with an outer surface of the casing or the tip end surface thereof is recessed interior of the outer surface, and wherein the tip end surface thereof is provided with an engaging part which is insertable/removable to/from an engaged part provided on a driven member.