We analyzed long-term oceanographic data (1997-2022) from the eastern Japan Sea and present observational evidence of overturning circulation―specifically, the advection of subducted waters―that connects the northern and southern regions across the polar front through the middle layer (density range around 27.30σ_θ). Low-salinity intermediate water (LSIW), characterized by a salinity minimum, flows under the influence of the eastward Tsushima Warm Current, with new LSIW appearing on the western side of the study area at intervals of several years. High-salinity intermediate water (HSIW), characterized by a salinity maximum, exhibits a long-term decreasing trend in salinity and potential temperature (corresponding to an increasing trend in potential density), though the precision of AUTOSAL measurements was insufficient for a detailed assessment of salinity accuracy. In this study, we examined not only the isopycnal structure of HSIW but also the horizontal distribution of apparent oxygen utilization (AOU) within the isopycnal layer between LSIW and HSIW (approximately 27.30σ_θ). AOU was used for two key reasons: (1) the relationship between preformed phosphate (preformed PO₄) and AOU suggests that mixing between HSIW (or waters in the density range around 27.30σ_θ) and other water masses is relatively weak, and (2) spatial variations in AOU allow for the identification of advection direction. Our results reveal a horizontal AOU gradient, with low AOU (indicative of newly ventilated water) in the north and high AOU (older water) in the south. This pattern was observed near the density range of HSIW during the early analysis period (1997-2001) and at a slightly lower density range in the later period (2002-2022). These findings provide evidence of a southward middle layer flow across the polar front.