The objective of this study is to calculate the tibiofemoral and patellofemoral forces at deep squatting and kneeling including seiza, which is the Japanese sedentary sitting. These postures are usually seen in daily life, especially in Japan or some regions in Asia or Arab. Thus it is expected to develop the artificial knee joint which is capable of making the posture, because the conventional prostheses cannot ensure deep knee flexion. We measured the joint angles of a lower limb and thigh-calf contact force at four postures. Then the tibiofemoral and patellofemoral joint forces were calculated, by using the force and moment equilibrium conditions on the muscloskeltal model at saggital plane. As a result, the thigh-calf contact force was the smallest at heel-contact squatting (0.60BW) and was the largest at heel-rise squatting (1.16BW). The knee flexion angle at these postures were almost the same, therefore the force might be effected the angles of hip and ankle joint. The tibiofemoral force was the smallest at seiza (0.64BW) and was the largest at heel-rise squatting (1.87BW). The patellofemoral force was also the smallest at seiza (0.74BW) and was the largest at heel-rise squatting (1.72BW). Neglecting the thigh-calf contact force, the joint forces were 3.8 times larger on average. Considering the thigh-calf contact force, which might be affected by not only knee joint angle but also hip and ankle joint angle, the knee joint force decreased extremely and the comparison between the postures underwent significant change.