To provide a folding door which prevents the gear parts of the gear hinge from being seen and prevents inserting a finger into the gear part while using a long gear hinge.

The folding door comprises a pair of folding door panels which integrally forms a gear part having an arc-shaped cross-section and forming a plurality of tooth grooves over the entire inside length of one stile composing the folding door panel, and a connecting plate which forms gear parts at both ends, each having an arc-shaped cross-section and forming a plurality of tooth grooves engaging the tooth grooves of the stile-side gear part. The folding door panels are foldably connected by engaging the tooth grooves of the gear parts at both ends of the connecting plate with the tooth grooves of the gear parts of the folding door panels respectively.