In a 4WD vehicle that is basically an FF type, a propeller shaft 4 includes, on a front end and a rear end thereof, respective driving force transmission devices 3 and 5. The propeller shaft 4 includes an electric motor 41, and when the electric motor 41 is rotated in a backward direction of the vehicle during a 2WD state, a hypoid ring gear 36 of a transfer mechanism 32 of the driving force transmission device 3 is moved so as to engage a clutch mechanism 31. By such an engagement, engine torque is transmitted to the driving force transmission device 5 via the propeller shaft 4. Thus, a hypoid ring gear 56 of a transfer mechanism 52 of the driving force transmission device 5 is moved so as to engage a clutch mechanism 51. Accordingly, a drive state is switched to a 4WD state.