Tripping over which is one of the causes of falling is caused by luck of recognizing obstacles. The 3D motion of five young adults was analyzed for stepping over an obstacle of 48mm high. Subjects who were more likely to trip over in usual activities had longer minimum distance between toe and obstacle. This result meant subjects who were more likely to trip over stepped over the obstacle more safety and contradicted the result of sensory test asked whether subjects were likely to trip over in usual activities. This suggested that one of the causes of tripping over was luck of recognizing obstacles in case of people who were more likely to trip over in usual activities. The effect of the visual sensation on human motion was also studied by using of eye tracking. Eye motion of five young adults was captured by eye tracking system during walking simulation of walking toward an obstacle using screen image.Subjects who were more likely to trip over were less likely to gaze at an obstacle. This result supported the results of motion analysis. Those results suggested that one of the main causes of tripping over was luck of recognizing obstacles and that it was possible to extract the effect of recognition on human motion by using of motion analysis and eye tracking.