PURPOSE: To suitably check quickly a danger of brine counterflowing by an operator when a gate is not fully closed due to some reason as the sea water level becomes higher than the internal water level by detecting the danger before the brine concentration in the water of the dam is increased.

CONSTITUTION: In a river-mouth dam gate controller for controlling the water restricting gate at the dam, it detects the difference between the upstream water level 1 and the downstream water level 2 to monitor Hin≤Hout+α by a comparator 20 to fully close the gates 9, 10 when the value becomes smaller than predetermined one.