In quality control of industrial products, it is important that materials, parts, and design but also production processes themselves. Generally, most production lines have processes to confirm quality of works in former processes in everywhere. However, there are some processes of which quality cannot be confirmed in later processes. For example, to fix a part using some screws, procedure to move the part onto correct pace and order to fasten screws is defined in most process. However, detecting violation of the procedure by visual or another way is hard in later processes. Therefore, we have been developing a system that guarantees quality of a process by tracing motions of worker's arms and hands by using terrestrial magnetism sensors and acceleration sensors. As a prototype system, we developed a system that traces worker’s motion and judges his/her work is correct or not at a process to attach a fuel tank in automobile assembly factory. In this paper, based on days long examination of the prototype system we will describe a method to judge worker's motion and its evaluation.