To prevent such defectiveness as widening of leg members, and improve withstand load performance compared to a conventional folding leg structure by applying a load in the vertical direction on a leg member when the load is applied on the leg member.

An intermediate frame 6 is provided with a horizontal shaft part 7 rotatably shaft-supported by a shaft bearing means 5 to be raised or fallen to a back surface 2a of a seat part 2. A first leg frame 10 is connected by a first connection member 23 to be raised or fallen to the intermediate frame 6. A second leg frame 11 is connected by a second connection member 24 to be raised or fallen to the intermediate frame 6. In a usage condition, therefore, the middle frame 6 is set in a raised condition, while the first and the second leg frames 10, 11 are elastically locked by a first and a second lock pawls 8, 9 to be set in a perpendicular condition to the back surface 2a of the seat part 2.