PROBLEM TO BE SOLVED: To obtain a cut surface as planned in which an acetabulum can be changed in position to an ilium when performing an osteotomy technique for hip joints.SOLUTION: A bone-cutting aid member 1 is used in an osteotomy technique for bones constituting hip joints, and is shaped along the periphery of a protrusion where the acetabulum being an object to be cut is protruded from the ilium. The bone-cutting aid member 1 includes: a fitting surface 11 shaped to form a circular arc-shape in the visual recognition from the direction toward the periphery in the ilium, having a shape matching the shape of the iliac bone surface in the periphery, and fitted to the bone surface and a guide section 12 extended from the fitting surface in the direction where the protrusion is protruded from the ilium while the fitting surface 11 is fitted to the bone surface, and guiding the cutting direction by the cutting jig for use in the acetabulum cutting.
