To allow smooth development in developing a folded-up folding door.

A guide member 8 movable along a rail 1 is mounted into the rail 1. The guide member 8 is provided with an inclined groove 9, and a guide roller mounted to a door tip side folding door 11 is fitted into the groove 9. A hanging side shaft pin 13 of a hanging side folding door 10 is fixed to a bearing piece 4 mounted to the right end of the rail 1. When the door tip side folding door 11 is folded toward the hanging side folding door 10, it is inclined at an angle exceeding 90° to the rail 1. When force F is applied to develop from this state, the component force of the force F becomes force f to push the end part of the door tip side folding door 11 to allow smooth development.