A downsized differential gear unit designed to prevent transmission of an excessive torque. The differential gear unit comprises: a differential mechanism held in a casing; a ring gear mounted rotatably and reciprocatably on the casing; a friction mechanism that is arranged between the ring gear and the casing; and an elastic member that pushes the ring gear in an axial direction. A torque possible to be transmitted between the ring gear and the casing is set to a torque governed by a pushing force of the elastic member and the frictional force of the friction mechanism.