A work for fixing a bone by an external fixator is facilitated. The external fixator comprises a plurality of ball joints each comprising a plurality of pins inserted into a bone, a coupling member, a rotary member that fixes pins to the coupling member relatively, is coupled by the coupling member and forms a ball at least partially, a supporting member in which a first supporting member and a first screw for supporting the ball rotatably are formed, and a tubular fastening member that contains the ball, has openings at the opposite ends, is provided at one end with a second screw corresponding to the first screw, and is provided at the other end with a second supporting portion for pushing the ball against the first supporting portion when the second screw is driven into the first screw, wherein the pins and the coupling member are fixed to the supporting member and the rotary member.