To simplify a structure and reduce a cost by eliminating the need for synchronous controls or the like of both wheels by incorporating an electric motor into either of a pair of right and left wheels.

The pair of right and left wheels 51, 52 are installed on both end of an axle 50: the electric motor 5 and a first wedge roller type change gear 6 which has a one-way clutch function are incorporated into the wheel 51; and a second wedge roller type change gear 6a, which is not incorporated with an electric motor, has a one-way clutch function, and has the reverse rotative direction relative to the wedge roller type change gear 6, is incorporated into the other wheel 52. A transmission shaft 53 that transmits a drive force to the second wedge roller type change gear 6a of the other wheel 52 from the electric motor 5 of the wheel 51 is incorporated into the axle 50.