PURPOSE: To absorb elongation of a drive wire effectively by arranging an upper and a lower foldback guide member so that the spacing of one of them is movable, and equipping the movable foldback guide member with a moving force applying means for moving apart from the other foldback guide member.

CONSTITUTION: A pulley 8 is supported rotatably by a shaft 9 protruding to one face 2a side from the boundary of an approx. projecting part 3a nearer one side of the top of a supporting plate 2 with the other part, and a groove is formed at the peripheral surface of this pulley. A movable pulley 10 assumes approx. the form of a moon with upper chord viewed from the front, and a stepped part is formed in the middle of the chord situated above. One end of a compressive spring 21 is inserted into a receptacle hole 13 in this movable pulley 10, while the other is resiliently contacting a butt wall 19 of the above mentioned supporting plate 2. When a DC motor 36 rotates, one end 39a of a wire 39 rises or sinks depending upon the rotating direction of the motor, and a bracket 40 coupled elevates or falls along a guide space 27. If an elongation is generated on the wire 39, the movable pulley 10 moves downward to remove slack of the wire 39, and thus tightened condition is held.