The present study proposes to call “dense Soya Current Water (DSCW)” a heavy water mass (of a density greater than 26.75 σθ) that originates from the winter-mixed water of Japan Sea. This remains at mid-depth in summer and appears in the bottom layer on the offshore side of Soya warm Current (SC). To confirm the seasonal appearance of DSCW and its geographical relationship with other water masses, we examined historical hydrographic data in the boundary region between shelf and basin areas off Abashiri. We paid special attention to avoid dangerous spatiotemporal averaging operations that create pseudo water masses. During the sea-ice season that lasts from late winter to spring, SC transfers heavy DSCW to the shelf area rather than light Soya Current Water (SCW). In the summer that SC is strengthened, SCW is dominantly altered on the shallower shelf area, while DSCW migrates to the offshore bottom layer. On the other hand, DSCW in the basin area appears deeper than 300 m during summer. The transport and modification process of DSCW in this boundary region were investigated by research vessel observation in the summer of 2019. Hydrographic data showed considerable modification of warm DSCW via mixing with offshore Intermediate Cold Water (ICW) and significant downwelling of modified DSCW on the steep slope of basin area. Model experiments proved that DSCW originating from the mid-depth of the Japan Sea in summer could be expanded into the deep basin area of the Okhotsk Sea in a short period within a month. We also found that the volumetric transport of such DSCW accounted for about half of SC inflow transport. These results suggest the physical contribution of DSCW should be taken into consideration more positively when discussing the dynamic process of the SC flow field.