An installation apparatus for installing a wire harness over a fixed structural body and a movable structural body which is slidingly movable to the fixed structural body, the installation apparatus includes a case that receives the wire harness such that the wire harness extends in a sliding direction of sliding movement of the movable structural body, and is folded back into a generally U-shape; and a slider that holds a distal end-side portion of the wire harness extending from the folded-back portion toward a distal end of the wire harness, and is slidably supported on the case so as to move in the sliding direction of the movable structural body, and is operatively connected to the movable structural body so as to move in accordance with the sliding movement of the movable structural body. A slit is formed in a wall of the case, and extends in the sliding direction of the movable structural body. The slit is arranged so as to intersect the folded-back portion of the wire harness in view of above of the wall in which the slit is formed. The distal end-side portion of the wire harness is led out from the case through the slit.