The objective of our study was to estimate the knee joint force during deep knee flexion. The studies of estimating the knee joint force had already been taken place, however, most of their subject were level walking and little subjected deep knee flexion. During deep knee flexion, thigh and calf contact each other and the contact force might work as to reduce the muscle forces of a lower limb and the knee joint force. Therefore we measured the thigh-calf contact force and took it into consideration estimating the knee joint. Thigh-calf contact force was measured by putting a pressure sensor sheet between thigh and calf. The motion to be objected was standing up from squatting position. As a result, thigh-calf contact force was about 0.8[BW] maximum at squatting position, and the knee joint force was reduced about 30% maximum by considering the contact force, 5[BW] to 3.5[BW] at squatting position. And it was important that, although the thigh-calf contact forces varied much among examinees, the variability was decreased by settling them with reference to the maximum knee flexion angles of each examinee. It could be mentioned that the thigh-calf contact force might be taken into consideration when estimating the knee joint force during deep knee flexion.