A compact joint coupling for a prosthetic brace can maintain a fixed state and can accommodate bending motion of the brace. A rotor having a recessed groove formed therein is rotatably mounted on a central shaft between a first coupling base and a second coupling base. Protruding pieces on the second coupling base are adapted to fit slidably within the groove. In addition, the surface of the second coupling base is covered with a cover for operating an operation pin. The groove is adapted to hold a plurality of spheres. The spheres can be manually introduced into, and removed from, the groove. The spheres are adapted to slide within the groove as the brace is bent. A purpose of the spheres is to prevent the brace from being bent by more than a predetermined amount.