a restoring reverse rotational motion another end of the rolling tube 2 is restrained by a displacement device 120 thereby enabling the rolling tube 2 to be transversely displaced during the course of taking up or letting down slats the displacement device 120 and a threaded rod 121 are coaxially configured one end of the threaded rod 121 is joined to the right stop end member 12 and a working end of the threaded rod 121 is rotate connected to a screw hole 210 defined in a corresponding rotate connecting end 21 of the rolling tube 2 rotate connecting end 21 because of the rotational motion of the rolling tube 2 thus the rotate connecting end 21 is slide shear displaced on the threaded rod 121 through a displacement distance of l 2 another end of the rolling tube 2 is movably disposed on a slide shear portion 112 and the slide shear portion 112 is directly formed on the stop end member 11 or directly configured on a corresponding position of the top rail 1 the slide shear portion 112 provides for transversal displacement of the rolling tube 2 through a transversal displacement distance of l 1 and primarily supports the rolling tube 2 to maintain displacement along an axle center line the lengths l 1 and l 2 are identical the rolling tube 2 is subjected to a reverse rotational driving force from the drive device 3 and a helical spring 31 provides a restoring motive force for the rolling tube 2 one end of the helical spring 31 is joined to an interior of the rolling tube 2 and another end is joined to a fixing portion 111 of the left stop end member 11 referring to.