Roller blind with centring device for the tractionbar

One end of a length-adjustable traction rod, which has a center component and two end components guided in the guide grooves of the guide rails (13), is connected to a wind-up shaft. A centering device performs the center positioning of the center component of the rod between the two end components. A drive mechanism (45) moves the traction rod along the guide rails and rotates the wind-up shaft. The wind-up window shade includes a window shade web (24) fastened to one edge of the wind-up shaft which is rotatably seated. The guide rails with the guide grooves are laterally extended beside the window shade web.