PURPOSE: To prevent generation of unstable operation when an input current is low, and generation of overshoot when a condition returns to a normal one.

CONSTITUTION: An input current Ii is supplied to an input terminal 11, passing through a Zener diode 13, a resistor 14, and an input terminal 12. Voltage between both electrodes of the Zener diode 13 shows a power source voltage Vcc. When the voltage Vcc is increased to be a specified value or higher, conduction is carried out at a transistor 31 for applying the voltage Vcc to a power source terminal of a differential amplifier 15. When an input voltage Vi which is a dropping voltage of the resistor 14 is increased to be a specified value or more, a switch 35 is changed over by a switch controlling circuit 36 to supply the voltage Vi to the differential amplifier 15 instead of a voltage Vd. An output of the differential amplifier 15 is supplied to an electromagnetic flapper unit 17, and a pneumatic pressure Po is output according to the output. The pneumatic output Po is converted into an electric signal and supplied to the differential amplifier 15 as a feedback voltage Vf.