PROBLEM TO BE SOLVED: To provide an osteotomy operation instrument that can make a cut in bone while restraining an influence on soft tissues existing around the bone.SOLUTION: An osteotomy operation instrument 1 according the present invention comprises: an osteotomy blade 20 for cutting osseous tissues and a guard 40 arranged so as to overlap with at least a blade edge 21 of the osteotomy blade 20 when the osteotomy blade 20 is viewed from a side face.