PURPOSE: To reversely send the practical aperture of a regulating valve controlled by a DC control current from the control valve side to the control device side.

CONSTITUTION: A pulse signal is generated by an electric signal outputted from a displacement/electric conversion means 35 mechanically connected to the regulating valve 31 and superimposed on a DC control current and a signal expressing aperture is transmitted to a control signal transmission line 21. The control device separates and identifies the pulse signal. A power supply current for generating the pulse signal can be obtained also from the DC control current. Consequently an aperture signal can be reversely transmitted without providing a specific transmission line. The control device can execute highly accurate and sure control by using the aperture signal.