Optimal and quasi-optimal navigations of an AUV achieved by the minimum time guidance in undersea area of sea currents are presented. When an AUV is to transfer to a given destination within an area of sea current disturbance, ingenious guidance enables the minimization of navigation time. In this study, a numerical procedure to obtain solution of the optimal guidance law in undersea area of sea current disturbances is presented. Simulated optimal and quasi-optimal navigations by minimum time guidance are realized based on the dynamics of an AUV ldquor2D4rdquo. The r2D4 is a deep ocean exploring AUV developed by Institute of Industrial Science, the University of Tokyo. The developed algorithm never fails to derive the optimal headings for minimum time navigation, if only the distribution of current velocity in navigation region is deterministic.