This paper describes a computer sided support system for localizing the distal transverse screw hole in interlocking intramedullary nailing. It is required to localize the distal transverse screw hole under surgeries for femur fracture. Conventional X-ray CT method can find the hole in the CT table. In it, when we move patients from the CT table to the surgery table, the exact potion is missed. Therefore, a real time determination of the hole position is required to enhance the quality. This paper solves the problem by using an ultrasonic system aided by fuzzy logic. First, we show the details of the equipments. Second fuzzy inferences effectively determine the fuzzy degree being the screw hole inserting to cattle femur. Finally, the experimental result shows that our system can determine the screw hole with 1.0mm of error.