The end of "the Cold War" in 1991 activated the studies of the oceanic circulation of the Japan/East Sea (hereafter, JES) through international cooperative studies between countries surrounding the JES, clarifying many interesting aspects of the northern part of the JES, which have been masked by "the iron curtain" during the war. The existence of a large cyclonic circulation in upper 1000 m was confirmed north of $40^{circ}N$ and east of $132^{circ}E$ in the Japan Basin by hydrographic surveys and PALACE float experiments. The low salinity water along the Russian coast originating from the Amur River feeds the subsurface low salinity core along the Subpolar Front corresponding to the southern boundary of the circulation as well as the subsurface salinity minimum (JES intermediate water) in the Tsushima Warm Water region. ARGOS buoys tracks suggest additional wind driven gyres such as a cyclonic gyre northwest of Hokkaido and an anticyclonic gyre in winter west of the large cyclonic gyre mentioned above. These gyres are generated by the wind stress curl in winter influenced greatly by geographical features of the northern coast of the JES. Numerical model studies reproduced successfully these aspects in the upper layer of the JES as well as many other interesting phenomena such as the branching of the Tsushima Current, the formation of intermediate waters, Karman vortex in the lee of the Tsushima Islands. Long-term deep current measurements suggest that strong cyclonic circulations (about 5 cm/s) follow the slope region of the deep three basins, whereas strong variability exceeding 30 cm/s episodically are dominant in the central flat area of the deep basins except several exceptions. This episodic strong variability is closely related with meso scale eddies found frequently in the sea surface temperature field by satellite. The JES has experienced a monotonic warming and structural changes as dramatically revealed in deepening of dissolved oxygen minimum by more than 1000 meters for the last several decades. An analysis of dissolved oxygen profiles shows that these changes resulted from a shift in its ventilation system: a slow down of a bottom water formation and an enhancement of intermediate water formation instead in recent years. It is remarkable to note that this shift is similar to a possible change in the global conveyor-belt system in the future associated with recent global warming (Manabe and Stouffer, 1998), thus strongly implying that the JES may serve as "a litmus paper" for the global warming.