Studies of sediment cores from the Sea of Okhotsk, which is characterized by seasonal sea ice, have shown a large variability of sea ice indicators during the glacial-interglacial cycles. In this study, we apply the relative abundance of the C37:4 alkenone to total C37 alkenones (%C37:4) as a molecular indicator of salinity and water masses to further investigate surface oceanographic condition in the Sea of Okhotsk in the glacial period. We found a large fluctuation of %C37:4 (4--35%) with higher values (20~35%) during the last glacial period and lower values (<8%) during warm periods, suggesting a decreased surface salinity in the glacial period. The variation of %C37:4 was found to be consistent with the ice rafted debris (IRD) in the sediment core (correlation coefficient (r2) between %C37:4 and IRD: 0.72). This suggests that the duration of seasonal sea ice was longer in the glacial period than today. The lowered surface salinity in the glacial might also have been seriously affected by the close-off of the shallow Soya Strait, which currently transports massive volumes of saline water from the Sea of Japan to the Sea of Okhotsk.