The paper presents real-time human motion analysis based on real-time inverse kinematics. Our purpose is to realize a mechanism of human-machine interaction via human gestures, and, as a first step, we have developed a computer-vision-based human motion analysis system. In general, man-machine "smart" interaction requires a real-time human full-body motion capturing system without special devices or markers. However, since such a vision-based human motion capturing system is essentially unstable and can only acquire partial information because of self-occlusion, we have to introduce a robust pose estimation strategy, or an appropriate human motion synthesis based on motion filtering. To solve this problem, we have developed a method based on inverse kinematics, which can estimate human postures with limited perceptual cues such as positions of a head, hands and feet. We outline a real-time and on-line human motion capture system and demonstrate a simple interaction system based on the motion capture system.