To achieve a structure of engaging a bone screw with a tool for ensuring the transmissibility of rotary torque and suppressing an increase in an outer diameter of a bone screw while facilitating setting of a posture of engaging the bone screw with a rotating tool.

A tool engaging part 12 of the bone screw is constituted of an engaging hole 12a opening at an end part of the proximal end part. Engaging regions 12x including corner parts 12h engaging the rotary tool are provided coaxially by using an axial line as a center at a pair of opposing positions provided on both sides with the axial line in between on an inner surface of the engaging hole. A pair of cylindrical surface-like transition regions 12y which have a diameter smaller than the corner part and are provided coaxially by using the axial line as the center are provided between the pair of engaging regions. The engaging regions have two adjacent corner parts 12h of a regular hexagonal cylinder circumscribing a virtual cylinder 12s along the transition regions.

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