An electro-pneumatic pressure converter including a switching valve having at least one plate-like piezoelectric element, which is supported at one end in a cantilever manner inside a sealed chamber and a distal end portion of which moves upon application of a voltage, at least two pressure nozzles which are opened and closed upon displacement of the piezoelectric element, and an output channel extending from the sealed chamber to an external portion; a pilot relay for converting a pneumatic pressure signal from the output channel into a pneumatic pressure signal; a pressure sensor for converting the output pressure signal of the pilot relay into an electrical signal; pulse width modulating means for supplying a pulse-width modulation signal corresponding to a difference between the electrical signal from the pressure sensor and an external input signal to the plate-like piezoelectric element of the switching valve; a power supplycircuit for obtaining a predetermined voltage from the external input voltage to supply a voltage to respective portions of the converter; and means for monitoring the external input voltage and inhibiting an output of the switching valve when the external voltage is decreased below a predetermined value smaller than a normal operating range.