This paper describes a physically precise spine model and its application to estimate three dimensional spine posture from the frontal and lateral views of a human body taken by two conventionalvideo cameras. In our method, rough position of each part is estimated by using a simple model applicable to images. Based on the obtained features from the fitting process of the simple model, the forces acting on the precise model are computed. The precise posture is estimated by deforming the model under these forces. Since the forces acting on intervertebral disks are easily calculated, the proposed method is useful for giving good suggestions to a user to achieve healthy and safe body posture. From the experimental results of application to actual human images, we confirmed that the proposed method estimated 3D posture of the human spine reasonably.