To provide a folding door hinge which is capable of being mounted always on a correct position on a door body.

In a folding door hinge, top and bottom outer surfaces of portions in the vicinity of a shaft inserting surface of hinge wings are formed into horizontal surfaces and an external dimension between the top and bottom horizontal surfaces in the perpendicular direction is made to match a dimension between a top and bottom inside corners of a substantially circular counterbore on a door body. In addition, a gap piece is provided, which has a vertical plate-like projection formed into a gap dimension between both door bodies and a flush surface portion. Further, a plurality of projections are provided to attachment surfaces in flange-shapes of the hinge wings. When the folding door hinge is mounted on the door body, the plurality of projections guide along an inside surface of the counterbore and the top and bottom horizontal surfaces of the hinge wings come into contact with the top and bottom inside corners of the counterbore. Consequently, the folding door hinge does not rotate with respect to the substantially circular counterbore of the door body, whereby a center line in the lateral direction of the folding door hinge is secured at a right angle to the door body with any mounting methods and hence the folding door hinge can always be mounted on a correct position.