The high-resolution analyses of fossil diatom assemblages in four cores recovered from the area off Sanriku Coast revealed the most complicated hydrographic history in this area. The fossil diatom assemblages changed in responding to the expansion of the warm-water eddies detached from the Kuroshio Extension and of the southward Oyashio Intrusion. Diatom abundances are higher in the cores from near shore than in the area off shore. The diatom assemblages were affected more seri- ously by the environmental changes in the cores from near shore. The diatom assemblages were also changed by the fluctua- tions at the intervals of ~40-kyr corresponding to the obliquity (tilt) band variations among the Earth's orbital parameters and ~60-kyr intervals suggesting changes in the sea level due to glacial/interglacial phases, with secondary and smaller fluctuatio ns at ~20-kyr intervals corresponding to the precessional changes. Diatom assemblages in the southern margin of the Perturbed Area have been controlled mainly by the Kuroshio-Kuroshio Extension, but in the northern margin they changed in responding to the mixing of warm-water eddies and Oyashio Intrusion.