Highly saline water (salinity-17 psu) in the lower layer of Lake Nakaumi often ascends along the lake bottom and flows into the Ohashi River. In this period, when the upsurge stage of internal oscillation with large amplitudes of 2-3 m appears along the western coast of the lake, a large amount of highly saline and anoxic water goes upstream through the Ohashi River. We carried out a 24- hour observation in the summer of 1996 to detect the internal oscillation and its effects on the vertical distribution of water properties, and found a dynamical relation between the internal oscillation and the anoxic water movement in Lake Nakaumi. We also performed sailing observations along the Ohashi River to examine the inflow process of anoxic water toward Lake Shinji and the dissolved oxygen consumption of the lower anoxic water. We found that the dissolved oxygen concentration showed a value below 6.0 mg 1^-1 in the whole region of the Ohashi River during the observation period, and that a rapid fall in dissolved oxygen concentration did not occur while the anoxic water passed through the river from Lake Nakaumi. The lower water of Lake Nakaumi seems to recover the dissolved oxygen concentration due to vertical mixing with the upper fresh river water at the shallow eastern mouth of the Ohashi River. It then moves upstream through the river with almost no consumption of dissolved oxygen.