Chest CT images obtained by CT scanner have drawn a great interest in suspicious region detection. However, mass screening based on CT images leads a considerable number of images to be diagnosed. CAD system for lung cancer that detects tumor candidates at early stage from CT images has developed. In July 1997, clinical trial using first version of the CAD system in Anti-Lung Cancer Association (National Cancer Center Hospital and National Cancer Hospital East) started. As following stage, clinical trial using second version of the CAD system that supports comparative reading function started in April 2002. We expect that CAD system reduces diagnosis time and increase the reliability. In this paper, we describe the clinical trial results using the CAD system from July 1997 and the detection results of the CAD system for supporting mass screening in a prospective study. The results show that the CAD system improves diagnostic accuracy and throughput. And we describe the future of CAD system.