A method and a system for cutting the knee joint using a robot. A cantilevered cutter is introduced into the knee joint from inside and outside of upper and lower bones of the knee joint to realize a tunnel cutting technique in which cutting is conducted such that tunnels are defined in bone and remnant bone is cleared. The cutter includes a shaft in which a substantial axial portion is fitted into a sleeve and a remaining axial portion serves as a cantilever extending out of the sleeve, and a head which is formed at a distal end of the shaft. A length of the cantilever and a diameter of the head are determined to have minimum sizes as long as the head can be introduced into bone in such a way as to define a tunnel and can be moved in the bone while cutting the bone.