When a fitting piece slips out of a rail groove of a guide rail in connection with an excessive load on a shutter curtain is restored to the rail groove, the restoration is reliably set and noiselessly carried out. A posture holding bar and fitting pieces, which are spaced from both right and left sides of the posture holding bar at a predetermined interval, are provided at the lower end portion of the shutter curtain. A first guide body guides the posture holding bar to the opposing site to the rail groove in the process of an opening/closing operation of the shutter curtain. A second guide body guides the fitting piece at the lower end portion of the shutter curtain to the opposing side to the rail groove in conformity with a timing at which the posture holding bar is positionally regulated to the opposing side to the rail groove by the first guide body is provided at the upper side of the rail groove constituting the guide rail.