Abstract: A water level regulating system controlling the water level of a head tank of each of a plurality of hydroelectric plant installed in cascade along a stream by using an opening of guide vanes of a hydraulic turbine as a manipulated variable, and the water level of the head tank as a controlled variable. Each hydroelectric plant has a water level regulator constituting the water level regulating system. Each of the water level regulator of an upstream hydroelectric plant and the water level regulator of a downstream hydroelectric plant has a device varying PID parameters for controlling the water level in accordance with the opening of the guide vanes, and a device varying a reference water level to a set water level.