To prevent the slipping-out of a screen even when a fixing screw is loosened or insufficiently fastened, and properly hold it in a structure for nipping the down protruding piece of an upper frame by slipping-out preventing tools mounted on both ends of the upper rail of the screen by fixing screws to prevent the slipping-out of the screen.

The body part of a slipping-out preventing tool 10 is housed within a right-angled sectional flange 6 opened in the upper part which is provided on the interior side of an upper rail 2, and a mounting part under the body part is passed through the notched hole 7 on the bottom part 6a of the flange 6 and protruded down. It is mounted by a fixing screw 4 through the slot of the mounting part. The body part has fins 14 on both sides, and each fin is brought into contact with the bottom part of the flange, so that the slipping-out preventing tool is held in a prescribed position by the fin even when the fixing screw is loosened.