By using all Japanese hydrographic data obtained south of 55°N in latitude in the Sea of Okhotsk, we compile a data set for this study. Our investigations with these data show that a large quantity of relatively cold, fresh, oxygen-rich homogeneous intermediate water with a density range of 26.8 to 26.9 σθ exists in the Kuril Basin; we name this intermediate water the Kuril Basin Intermediate Water (KBIW). Results of isosteric analyses show that a possible origin of KBIW is located in a region off the Hokkaido coast in the southwestern Sea of Okhotsk. We suggest that KBIW is produced by the isopycnal mixing of relatively dense Soya Warm Water, which appears only in early spring, with the colder, fresher water which originates from the northwestern Okhotsk. Finally, we demonstrate that KBIW may be a source water of the North Pacific Intermediate Water (NPIW) because it has the same density range and water properties.