PURPOSE: To perform reliable setting of a dislocation preventing device without being left non-set and to simplify structure.

CONSTITUTION: An automatic dislocation preventing device comprises a bottom base part located to the bottom part of an upper groove, a side base part erected integrally with the side part of the bottom base part, a dislocation preventing piece located horizontally rotatably around a longitudinal shaft located closer to the side part of the bottom base part, a spring through the force of which the dislocation preventing piece is rotated to the side wall side opposite to the longitudinal shaft side of the upper groove, and a stopper extended from the bottom base part and located in a state to levitate from the bottom part of the upper groove and composed of a resilient piece to which the dislocation preventing piece is locked. By pressing down the stopper to the upper rail of a frame body, the dislocation preventing piece is dislocated from the stopper and rotated through the force of the spring. Meanwhile, the dislocation preventing piece is located between the upper rail of a screen and the bottom part of the upper groove.