A driving mechanism is arranged such that at least a part is contained within a space corresponding to the external shape of a driving motor, that is, within a space corresponding to a distance in the radial direction of the driving motor. The driving mechanism comprises a pinion, a transmission gear, a ring gear that forms a fixed axis, and a carrier. The pinion is connected to a rotary shaft of the driving motor. The transmission gear has a disk gear portion that meshes with the pinion, and a sun gear portion that is located on the same side as the pinion with respect to the disk gear portion. The carrier has a planet gear and rotates with a drum.