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The sit-to-stand (STS) motion is difficult for elderly people in need of care. These individuals generally require assistance from two caregivers in order to go to the toilet-one to help them maintain a standing posture, and the other to assist them in undressing and dressing. Therefore, a robot that could help only one person assist in elder care is very desirable. In previous studies, robots have been designed to assist elderly users to maintain a standing posture by supporting part of their weight during STS motion. However, these robots are big and heavy. In addition, they apply pressure to one part of the body, potentially causing the user discomfort. Here, we consider knee instability or buckling, which is a sudden knee flexion. This represents a situation that one caregiver cannot handle alone. In this study, to prevent knee buckling, we developed a small, lightweight robot knee orthosis. First, we analyzed knee buckling during STS motion, and demonstrated that one caregiver is sufficient when knee buckling is prevented. Next, we developed a robot knee orthosis with a ratchet mechanism and control system to prevent buckling. Finally, we conducted experiments with the developed device, and confirmed its effectiveness in preventing falls.