A power unit 16 includes an armature 38 rotated always in unison with a cable drum 17 and movable in an axial direction of a drum shaft 25, a flange 39 always connected to a motor 28 side, and a magnetic coil portion 37 bringing the armature 38 into abutment against the flange 39. The armature is disposed adjacent to a side of the drum rim body 23 of the cable drum 17, the magnetic coil portion 37 is disposed on the other side of the armature 38, and the flange 39 is disposed between the magnetic coil portion 37 and the armature 38.