Many kinds of lumbar diseases cause lumbar segmental instability and/or limitation of motion. But, there is no reliable method for measuring accurate trunk motion. In order to measure relative trunk motion respect to the pelvis in vitro, external linkage type of spine motion analyzer was developed. Special programs for calculation of the relative angular motion and graphical display were also developed. The developed device was tested to compare spine motion pattern between 15 normal volunteers and 18 lumbar fusion patients in flexion, extension, lateral bending and axial rotation. It was rather simple and reliable method for spine motion analysis. And it was possible to measure not only angular displacement but also angular velocity and angular acceleration. There were more coupling motions in lateral bending and axial rotation than in flexion and extension. Compared to the normal subjects, patients showed different angular motion, especially in the angular acceleration. Optimal trajectory of the trunk motion derived from mathematical model in flexion and extension matched well with measurement for normal subjects.