Eight sets of coastal acoustic tomography (CAT) systems were deployed during November 29 to December 10,2002 at the coasts on both sides of Tokyo Bay to measure tidal current structures at 15-min interval.Sound transmission across the Tokyo Bay (between Yokohama and Chiba)was successfully traced,even under severe interference from ship generated wakes and bubbles.Tidal current fields changing from northward to southward flow are well reconstructed by the inverse analysis of travel-time difference data for a period with the best sound transmission condition.It is suggested that the CAT is the most powerful tool to continuously map tidal current fields in the coastal seas with heavy shipping traffic and fisheries activity.