To provide a coupling structure of a bone fixation system, the coupling structure being capable of preventing screw coupling from loosening even when axial force reduces and capable of inhibiting dislocation of a fractured bone segment after a surgical operation.SOLUTION: A bone fixation system 10 includes: a first fixing tool 11 comprising a first engaging connection part 11a； and a second fixing tool 12 comprising a second engaging connection part 12bcapable of engaging and connecting with the first engaging connection part 11ain a mode of permitting relative helical movement when a depth of screw coupling increases or decreases. The first engaging connection part 11aand the second engaging connection part 12bare configured to be capable of engaging and connecting with each other at an engaging position corresponding to a plurality of holding positions arranged along a direction of the relative helical movement in a mode of generating holding force between the first fixing tool and the second fixing tool at the engaging position and relatively move while performing engaging/disengaging operation across a plurality of engaging positions with the relative helical movement.SELECTED DRAWING: Figure 1【課題】骨固定システムにおいて、軸力が低下してもねじ結合の緩みを防止でき、手術後の骨折部の転位を抑制できる連結構造を提供する。【解決手段】本発明に係る骨固定システム１０において、第１の固定具１１は、第１の係合結合部１１ａ２をさらに備え、第２の固定具１２は、前記ねじ結合の深さが増減する際の相対的らせん運動を許容する態様で第１の係合結合部１１ａ２に対して係合結合可能な第２の係合結合部１２ｂ１をさらに備え、第１の係合結合部１１ａ２と第２の係合結合部１２ｂ１は、相対的らせん運動の方向に沿って配列された複数の保持位置に対応する係合位置において、保持位置で第１の固定具と第２の固定具の間に保持力を生じさせる態様で相互に係合結合可能に構成されるとともに、相対的らせん運動に伴って複数の係合位置の間を係脱動作しながら相対移動する。【選択図】 図１
