To improve the folding and operation properties of a folding door and reduce a dead space.

Both end parts of a door body support arm 8 are connected with parts close to end parts on an original hanging side of an arm receiver 3B and a door body 5 on the original hanging side which are formed in end parts of an upper rail 3 arranged in the direction of a bay in such a manner that they can turn around a vertical shaft freely. A substantially central part in the direction of width of the door body 5 on the original hanging side is engaged with the upper rail 3 in such a manner that it can slide and turn freely, and a pull framing 7A provided in an end part on a door tip side of a door body 7 on the door tip side in such a manner that it can turn freely is engaged with the upper rail 3 in such a manner that it can slide freely.