PROBLEM TO BE SOLVED: To provide a modular knee implant system capable of exchanging a constituting part of systems between the systems. SOLUTION: In the modular knee implant system, a surgeon can use several kinds of distal thigh bone implant constituting parts and a diaphysial end constituting part selectable from several kinds of stem extension parts. The diaphysial end constituting part can be formed of an all purpose part usable with all kinds of distal thigh bone implant constituting parts by using an adapter. A stem extension part having another kind of connector can be used with the diaphysial end constituting part by a second adapter. A distal Morse taper post coincident with the distal thigh bone constituting part having a proximal Morse taper hole, can also be arranged in another diaphysial end constituting part. The diaphysial end constituting part has an outside surface constituted so as to maximize a contact area with a bone of a patient, and can have the surface finish for promoting growth of the bone over the substantially total length. COPYRIGHT: (C)2005,JPO&NCIPI