PURPOSE: To simply cover an opened groove, by fitting a movable U-shaped rail into a recess part of a recess-shaped base frame for the rail in which a guiding lower rail is laid, being turned upside down according as a shutter is traveled for opening or shutting.

CONSTITUTION: When a U-shaped rail 11 is exposed by opening a shutter 15, the rail 11 fitted into a base frame 12 for the rail 11 is taken out of the base frame 12. After that, the rail 11 is refitted into the base frame 12 in the state that a bottom plate 11b thereof is turned upside down and is turned into the top surface, and an opened groove is covered. In the case where a guiding lower rail is formed in curve shape, a U-shaped rail is divided into symmetrical rail members at the central part thereof. Each of the U-shaped rails divided is turned upside down, the symmetirical parts are alternately varied and are connected, and the entire U-shaped rails are made in curve shape connected upside down. Furthermore, when the shutter 15 is traveled, the rail turned over is turned upside down and is restored to the original state of U-shape, and the shutter 15 is shut. In this way, the covering of the opened groove is made in simple structure, dust can be prevented from entering forcibly, and the shutter 15 can be safely traveled.