Due to the effects of the pandemic situation, remote work hours are increasing, and people are working at their desks for long periods. Therefore, there is concern about physical fatigue due to the accumulation of physical loads caused by continuous seating posture. Therefore, a system that monitors sitting posture and provides information to the seated person is expected to change behavior to reduce physical loads. In this study, we have developed a system that estimates musculoskeletal loads by inputting the sitting posture measured by unconstrained image acquisition. Calculation of musculoskeletal loads needs skeletal posture and reaction forces from the chair. In the system, reaction forces from the chair were estimated based on the balance of external forces from the measured skeletal posture. Using this system, we calculated the musculoskeletal loads during prolonged sitting from the measurement results of skeletal posture during continuous desk work and analyzed the relationship between the change in musculoskeletal loads and postural change. As a result, the possibility of the system was validated by analysis of the relationship between internal loads and posture.