Transtibial amputees have lost their ankle-foot complexes and gastrocnemius muscles, which also affect the knee joint as well as the ankle. We optimized the engagement time and spring stiffness such that a transtibial amputee musculoskeletal model can track the biological knee and hip joint torque profiles. Moreover, we have developed the prosthetic device for transtibial amputees, which is composed of a knee flexor and a powered ankle-foot prosthesis. The knee flexor works equivalently to biological gastrocnemius muscles. Pilot Trial was conducted on a bi-lateral transtibial amputee. In the trial, we measured his energy expenditure for his self-selected walking speed with our device and conventional ankle-prosthesis. The result shows reduction in his cost of transport.