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Surgical reconstruction of bony defects such as Rotational Acetabular Osteotomy (RAO) in the orthopedics field involves the operation of bony cutting using surgical saws or drills. Operators have to incise skin and open the muscular system under anesthesia to make the bone visible, and this causes patients great pain and marks after the operation. So a system of bony cutting using focused ultrasound without incising the skin has been developed. The ultrasound energy and the high pressure caused by cavitation generated by the minus vapor pressure ablates the focus area of the bone. An experiment of ablating the surface of a pig's bone by ultrasound was carried out in various irradiation conditions. At a high oscillating frequency, in the case of continuum waves, the mark of ablation without thermal damage was found at the focal point and the size was the same as the convergence beam area of the ultrasonic wave. In addition, ultrasonic propagation in the living body which consists of multiple organizations was analyzed using the time-domain finite element method. In comparison with the experimental result, some important parameters relating to the cause of thermal damage at the focus on the bone were specified.