A spinal fixation tool holding device for holding a spinal fixation tool, the spinal fixation tool including a bone engagement portion provided at a distal end portion and engageable with a bone, and a head portion provided at a proximal end portion, the head portion having a cylindrical lower portion that holds the bone engagement portion, and an upper portion extending upward from the lower portion and having an outer wall and an inner wall having an identical central axis to a central axis of an outer shape of the lower portion, the upper portion having a pair of slots which are opened in a direction perpendicular to the central axis in parts of the outer wall and the inner wall around the central axis, wherein a circular cylindrical portion forming an outer periphery of the lower portion includes an outer circumferential circle that defines a maximum diameter of the head portion, the holding device comprising: a cylindrical main body portion having an outer cross-section equal to or less than a cross-section defining the inner wall of the upper portion; and a pair of locking pieces protruding outward from the distal end portion of the main body portion in the central axis direction along a direction perpendicular to the central axis at the pair of slot positions while the main body portion is positioned within the inner wall of the upper portion when viewed from the central axis direction, wherein the pair of locking pieces have engagement portions that are engaged with side walls joining the outer wall and the inner wall of the head portion while being positioned inside the pair of slots, characterized in that the pair of locking pieces each have an arcuate shape extending along the outer circumferential circle, which defines the maximum diameter of the head portion, while being positioned within the pair of slots. the spinal fixation tool including a bone engagement portion provided at a distal end portion and engageable with a bone, and a head portion provided at a proximal end portion, a bone engagement portion provided at a distal end portion and engageable with a bone, and a head portion provided at a proximal end portion, the head portion having a cylindrical lower portion that holds the bone engagement portion, and an upper portion extending upward from the lower portion and having an outer wall and an inner wall having an identical central axis to a central axis of an outer shape of the lower portion, a cylindrical lower portion that holds the bone engagement portion, and an upper portion extending upward from the lower portion and having an outer wall and an inner wall having an identical central axis to a central axis of an outer shape of the lower portion, the upper portion having a pair of slots which are opened in a direction perpendicular to the central axis in parts of the outer wall and the inner wall around the central axis, wherein a circular cylindrical portion forming an outer periphery of the lower portion includes an outer circumferential circle that defines a maximum diameter of the head portion, the holding device comprising: a cylindrical main body portion having an outer cross-section equal to or less than a cross-section defining the inner wall of the upper portion; and a pair of locking pieces protruding outward from the distal end portion of the main body portion in the central axis direction along a direction perpendicular to the central axis at the pair of slot positions while the main body portion is positioned within the inner wall of the upper portion when viewed from the central axis direction, a cylindrical main body portion having an outer cross-section equal to or less than a cross-section defining the inner wall of the upper portion; and a pair of locking pieces protruding outward from the distal end portion of the main body portion in the central axis direction along a direction perpendicular to the central axis at the pair of slot positions while the main body portion is positioned within the inner wall of the upper portion when viewed from the central axis direction, wherein the pair of locking pieces have engagement portions that are engaged with side walls joining the outer wall and the inner wall of the head portion while being positioned inside the pair of slots, characterized in that the pair of locking pieces each have an arcuate shape extending along the outer circumferential circle, which defines the maximum diameter of the head portion, while being positioned within the pair of slots.