In this study, the relationship between the behavior of fish school, Jack mackerel Trachurus japonicus and oceanic conditions was investigated in Tateyama Bay, Chiba Prefecture, Japan. We showed that the fish school entered from offshore to the inside of Tateyama Bay with the flood tide inflow of water masses having lower temperature and higher salinity at the bottom layer. The fish school appeared in the inside of Tateyama bay along with mixed water masses of Tokyo Bay, Sagami Bay, and the offshore Kuroshio water. The Kuroshio water mass was distributed over the depth from several tenth of meter to around 100 m depth off Tateyama Bay. These results indicated that the fish school behaves with a diurnal tidal cycle in accordance with the movement of the Kuroshio water, and as a result, Jack mackerel was fished near the bottom layer where the mixed water was flowed into the bay.