PURPOSE: To enable abnormality to be informed to the outside as well as to enable an electropneumatic position to be safely operated in relation to abnormality of a valve opening detecting unit by switching the output of a pressure converter and the valve opening detecting unit by means of a comparison computer.

CONSTITUTION: An electropneumatic positioner converts a deviation signal Vd as the deviation between a voltage signal Vo corresponding to a current signal Io and a feedback signal Vr into a corresponding air pressure signal Po and controls the valve opening of a valve 27 of an adjusting valve 24. The valve opening is input as the displacement of a stem 26 into a valve opening detecting unit 28a and converted into a valve opening signal Vs. The valve opening signal Vs is fed back as the feedback signal Vr through a switch SW and a computing circuit 29. The pressure signal Va and the valve opening signal Vs are input into a comparison computior 30, and the comparison computer 30 compares these signal with each other, and the switch SW is switched by a control signal Vc. An abnormality judging circuit 31 detects the state of the control signal Vc1 of the comparison computer 30. When the detected state exceeds the specific range, an alarm signal AL1 is superimposed on a transmission line for receiving the current signal Io, so as to recognize the feeding abnormality.