A position locating instrument for accurately determining the location of a cutting plane includes a base connected to means capable of determining the position of the instrument. The base includes bent blades 28 A to 28 D that are flexible enough not to prevent the insertion of plate 24 into opening 36 of a bone cuffing guide. Bent blades 28 A to 28 D are however rigid enough to maintain lower surface 25 of plate 24 pressed against lower surface 38 of opening 36, with the free ends of bent blades 28 A to 28 D bearing against upper surface 39 of opening 36. Bent blades 28 A to 28 D have a sufficient stiffness to provide a sufficient force to oppose displacement of plate 24 that is inserted in opening 36 by pushing lower surface 25 of plate 24 onto lower surface 38 of opening 36. Play between plate 24 and opening 36 are suppressed. By determining the position of plate 24, the position of the cuffing plane is thus directly and accurately obtained.