Tunnel management system (TMS) is a software system that stores data on current tunnel conditions. The system designs repair strategy necessary to keep waterway tunnels in proper condition. The TMS database includes eye observation records, boring data, GPR-acquired information, etc. GPR has been widely used in Japan to disclose unseen objects behind the lining of waterway tunnels. Application has been limited, however, to comparatively larger diameter tunnels. A GPR rapid-survey system for small-diameter tunnels has been developed to fulfill the need for database completion of the TMS. Through application to many tunnels, GPR has been found effective in terms of obtaining knowledge objects behind the lining. Through theoretical speculation and experience, the interpreting effort has revealed many unseen events such as void, collapsed soil, and water accumulation. Such GPR information gave firm basis to learning the reason of faults (such as cracks, which appeared on the surface), evaluating the degree of risk for collapse, and determining repair methods.