An osteosynthesis device is provided, which, while being a single device, is wide in a range of fixing positions or fixing methods that can be selected in a repositioning operation, enables firm fixation of an affected portion, and alleviates feeling of discomfort felt at the affected portion after the operation.An osteosynthesis device D1 is a device used, in case of fracture of an olecranon side of an ulna, to fix and reposition respective bone fragments and includes an intramedullary nail 1 inserted into the ulna 3, an end plate 2a mounted on an outer surface of the ulna at the olecranon 31 side, and a threaded screw N1 arranged to screw on the end plate to the intramedullary nail. Screws S are used in combination with the osteosynthesis device D1 to fix, etc., the inserted intramedullary nail 1 onto the ulna 3. The intramedullary nail 1, which is a round rod body having a tapering tip end portion and has a thickness enabling entry into the marrow of the ulna and a predetermined length, includes a threaded hole 111 formed in a base end surface 101, long holes 114 and 115 formed at a base end side with respect to a central portion of a circumferential surface 102, and circular holes 112, 113, 116, 117, and 118 formed at predetermined parts between a tip portion side and the base end side of the circumferential surface 102.
