To prevent a foreign substance from invading into a guide rail.

A flap member 40, which can swing in the width direction of a slit 23 and is located at a cover position covering almost the whole length of the upper part of the slit 23 in the normal state and can swing to a retracting position leaving from the upper part of the slit 23, interacting with a fastening band sent out of the slit 23, is provided over a slit 23 of the guide rail 21 under an exhaust port 15 so that the foreign substance can be discarded to a fastening guide 31 side.