To transmit rotation from a motor to a wheel side by transmitting auxiliary power to a wheel via a rotor having a diameter equal to the rim of the wheel and greatly reducing the rotation from the motor with a simply structured reduction gear.

A power transmission mechanism for transmitting the rotation of a driving mechanism to a rear wheel is composed of a second transmission belt 31, a third follower pulley 32 and a pulley stay 33. Herein, the third follower pulley 32 has a diameter equal to the rim 4 of a rear wheel and, in order to rotate concentric-circularly with the rear wheel in the left side of the same, the inner circumferential surface of the third follower pulley 32 is connected to the inner circumferential surface of the rim 4 by a plurality of pulley stays 33. In addition, when the third follower pulley 32 is connected to a second follower pulley 24 via the second transmission belt 31, the rotation of the second follower pulley 24 is transmitted through the second transmission belt 31 to the third follower pulley 32 and a rotating number is reduced by 1/7. Thus, rotation transmitted from the motor is greatly reduced by a simply structured reduction gear and then transmitted to the wheel side.