To reduce energy loss to increase power transmission efficiency, as well as to suppress unusual sound resulting from whirling vibration. A driving force transmission mechanism includes a drive unit that generates a rotation force and consists of a motor, as well as an independent rotating mechanism that is disposed between the drive unit and one of a pair of left and right gear mechanisms so as to be connected to the output shaft of the drive unit and has greater kinetic energy than the rotating parts of the gear mechanisms. Flexible shafts that rotate by smaller kinetic energy than any of the kinetic energy of the drive unit, the kinetic energy of the rotating mechanism, and the friction forces and damping forces of the rotating parts of the gear mechanisms connect between the drive unit and rotating mechanism, between the rotating mechanism and one gear mechanism, and between the drive unit and the other gear mechanism.