The spinal fixation adapter (6) includes an engaging portion (6a) projecting downward to engage with an engaged portion of the bone engaging member, and extending upward from the engaging portion (6a). In the state in which the engaging portion (6a) is engaged with the engaged portion of the bone engaging member in the housing portion of the head portion, the spinal fixation tool is brought into contact with the inner wall of the head portion directly or indirectly. And a restricting portion (6b) for restricting three-dimensional swinging of the bone engaging member integrated with the adapter (6).脊椎固定具用アダプタ（６）は、骨係合部材の被係合部に係合するよう下方に突出した係合部（６ａ）と、係合部（６ａ）から上方に延出され、ヘッド部の収容部内において係合部（６ａ）が骨係合部材の被係合部に係合された状態で、ヘッド部の内壁に直接的または間接的に当該内壁に接することにより、脊椎固定具用アダプタ（６）と一体となった骨係合部材の３次元的な揺動を規制する規制部（６ｂ）と、を備えている。
