To provide a driving force transmitting device capable of improving the fuel economy in non-driving an auxiliary driving wheel, and reducing the cost.

In this driving force transmitting device, the torque is transmitted to the auxiliary driving wheel connected and fixed to axles 40, 41 from a speed reduction mechanism 21 through a differential gear. A clutch 22 as a two-way clutch is mounted between a final speed reduction part of the speed reduction mechanism 21 and the differential gear, and an input gear 46 forming a secondary speed reduction part 29 is mounted on an outer periphery of the differential gear through bearings 45, 49. When the clutch 22 is disengaged, the torque from the auxiliary driving wheel side is not transmitted to the final speed reduction part of the speed reduction mechanism 21. That is, the speed reduction mechanism 21 is not operated.