A first table defines the relationship between the sliding resistance index μk of a packing gland and the ambient temperature for each type of the packing gland is provided. A second table defines control parameters corresponding to hysteresis levels for each size of the setting/operating device is provided. By repeating, at regular intervals, the acquisition of the sliding resistance index μk at present corresponding to the ambient temperature at present and the type of a packing gland from the first table and the selection of the control parameter corresponding to the size of the setting/operating device and the hysteresis level obtained from the sliding resistance index μk at present from the second table, the valve opening of the regulating valve is controlled using the selected control parameter.