A horizontal wall is formed with a notch groove. The notch groove is formed by cutting a tip end of the horizontal wall on the side of the vehicle body substantially along an approaching/separating direction between a vehicle body and a slide door. A connecting unit which turnably connects a plate and a roller holder of a connection assembly to each other is movably inserted into the notch groove. The plate and the horizontal wall of the bracket are fastened to each other in a state where they are superposed on each other.