To provide a new folding door hinge structure of uniaxial structure capable of eliminating the slippage between panels in a folded and closed state to realize satisfactory running property.

U-shaped sectional vertical frame members 2 are mounted on the mutually opposed side end surfaces 11 of the panels 1 so as to be opposite to the respective openings 23, and a semicircular sectional clearance regulating member 3 is provided between the opening parts 23 of the vertical frame members 2 substantially extending over the whole length of the vertical frame members 2. The vertical frame members 2 are foldably connected to each other by a hinge part 4 provided on the flat part 31 of the clearance regulating member 3, and the clearance between the vertical frame members 2 caused in the folding is shielded by the curved surface part 32 of the clearance regulating member 3.