Lake Takahoko, the brackish lake located in Aomori Prefecture, Japan, is connected to the Pacific Ocean in the east through a channel, which has narrow mouths at both ends. The lake is partitioned almost at the center into west and east sides by a levee with water gates, and one of which is usually opened. In order to clarify the inflow and outflow rate of water and salinity, and seawater exchange rate in the lake, the salinity, water current and water level were observed at stations set up at sea side (St. B) and lake side mouth (St. C) of channel and the water gates of levee (St. G) from August to November 2014. Mean inflow rate of water at Sts. B, C, and G were estimated at 6.4 × 105, 1.8 × 105 and 8.7 × 104 m3 d–1, respectively. The inflow rate of water at Sts. B and C periodically varied with large amplitude during spring tide and small amplitude during neap tide, while that at St. G did not show such periodicity. Means of daily seawater exchange rate at Sts. C and G were estimated at 0.40 (variation value 0.04–0.61) and 0.33 (variation value 0.12–0.53).