Abstract: The safety of control may be secured by decreasing error in the valve position detection to increase the accuracy of flow control and stopping the control function when the valve is in an abnormal condition, so as to prevent any malfunction. The valve position is controlled so that the valve opening rate may correspond with the intended opening rate. When judgement is made to determine that said valve opening rate corresponds with the full-closed position, an output signal produced by the means for detecting the valve displacement when the valve is in the full-closed position is updated and stored as a new signal showing the valve full-closed position, and the intended opening rate of the valve is corrected with reference to the stored value.