A surgical instrument including a pre-cut trial mounted on a pre-cut line which is cut shallower than a genuine-cut line. The pre-cut trial is modeled after the genuine implant which is mounted on resected surfaces formed by performing genuine-cutting and comprised of an end resected surface, posterior and anterior resected surfaces, and posterior and anterior chamfer-resected surfaces on the distal end of a femur, and the pre-cut trial is mounted on the pre-cut line, thus observing the extended and flexed positions of the femur and the state of flexion thereof so as to make judgment if the genuine implant is appropriate.
