Several previous studies investigated comfort while soaking in hot bathwater from the perspective of thermal effects or physiology. However, few studies investigated bathing comfort from the perspective of biomechanics, although the biomechanical state in bathing, such as buoyancy on a human and the degree of muscle contraction is expected to be changed according to soaking postures. Indeed, only a few studies from the biomechanical viewpoint provided biomechanical models. In addition, the provide models were insufficient to discuss biomechanical loads in detail. The objective of this study was to evaluate soaking postures considering the effects of buoyancy, muscle contraction and passive elastic joint moment from the perspective of biomechanics and to evaluate biomechanical loads in detail. Soaking postures and reaction forces from the bathtub to a human were measured for ten healthy male participants and under two bathtub conditions (recent bathtub and conventional one). A three-dimensional motion analysis system and waterproofed three-dimensional force plates were used to measure the experimental data. A biomechanical model in which a human body was represented as a link of body segments was constructed. The torque due to buoyancy and passive elastic joint moment were considered in the model. The result showed that all the torque components due to buoyancy, gravity, reaction forces, and passive elastic joint moment contributed to the joint torques and each torque component was changed between bathtub conditions. In addition, joint torques on the ankle and knee joints in the recent bathtub were significantly smaller than those in the conventional bathtub. These results suggested that the bathing posture in the recent bathtub was more comfortable than that in the conventional one. Furthermore, the difference in joint torques between bathtub conditions suggested a potential benefit for designing bathtub shape, in which know-how of developers or subjective assessment is relied previously, with quantifying from biomechanical viewpoint.