PURPOSE: To keep a constant water level even when the change of flow-out rate is large in a secondary water tank with a small capacity by feedfoward- compensating a water level control system, and constituting an opening degree control system of an open loop.

CONSTITUTION: An adjusting gate 3 which is driven by an electric motor is provided at a water guide between a primary water tank 1 of an upstream side and a secondary water rank 2 of a downstream, and the water level of the secondary water tank 2 is controlled so as to be constant by the control of the opening degree of the adjusting gate 3 by a controller 4A. In the controller 4A, a necessary gate opening degree (h) is predicted from a detection water level H1 of the primary water tank, and flow-out rate Q0 and set water level H2 of the secondary water tank by a prediction arithmetic part 12, the predicted value is added to the output of a water level control amplifier 7 as the feedfoward compensation, the output of a control relay 10 is controlled by the added result, and the gate opening degree of the adjusting gate 3 which is driven by a motor 11 is controlled.