To attain stable water level control by relaxing the secondary influence on the fluctuation of the water level of a pressure reduction well generated due to the movement of a control valve at a downstream side.
A main opening degree M1 is calculated by a PID arithmetic operation by using a water level signal in a pressure reduction well 5a to be controlled as an input value, and a correction opening degree M2 is calculated by the PID arithmetic operation by using a water level signal in a pressure reduction well 5b positioned at a very upstream side as an input value. Then, the product of a correction weighting factor n2 preliminarily calculated by a rule of thumb and the correction opening degree M2 is added to the product of a main weighting factor n1 preliminarily calculated by a rule of thumb and the main opening degree M1, and an operational target opening degree Mo of a control valve 7a is calculated.