To prevent a door forming a folding door from being easily bent about a connection member.

This folding door is formed by foldably connecting two doors 4 through a connection member 3. The vertical frame members for connection 5 close to each other of the doors 4 adjacent to each other are installed swingably on the connection member 3 through a hinge part 6. The hinge part 6 is formed by the combination of a first hinge component part 7 formed on the connection member 3 with a second hinge component part 8 formed on the vertical frame members for connection 5 of the doors 4. An engagement teeth 30 is provided in the connection members 3. An engagement claw 42 engaged with the engagement teeth 30 when two doors 4 adjacent to each other are arranged in series is installed on the vertical frame members for connection 5 of the doors 4.