Wheelchair falls have become a major problem for people with spinal cord injuries, resulting in serious accidents such as head injuries and fractures. However, electric wheelchairs are expensive, and wheelchair users who have some upper limb mobility may lose the opportunity to use their residual functions, which may lead to joint contractures and disuse muscular atrophy. The wheelchair user's ability to use the upper limbs is also affected. In this study, we proposed a system that measures the height and distance of obstacles using a camera and a cross line laser that can be mounted on a manual wheelchair, and warns the wheelchair user when the wheelchair goes over the obstacle at a height that may cause the wheelchair user to fall. We installed the system on a crawler robot and conducted an experiment to verify the effectiveness of the system.