PURPOSE: To prevent stray running of outputs, by performing control and operation under the state where nozzle back pressure is received and a nozzle holding means is contacted with a stopper, separating piezoelectric element flapper from the nozzle by a DC voltage, which is held, when input of a power source is OFF, and isolating the flapper from the stopper.

CONSTITUTION: When an input current Ii flows, a voltage, which is applied to a piezoelectric flapper 2, is increased to the maximum value. The flapper 2 is changed into the approaching direction to a nozzle 3, and nozzle back pressure is increased. Output pressure is increased accordingly, and a diaphragm is also changed in the approaching direction to the flapper 2. When the nozzle back pressure becomes specified output air pressure, the diaphragm 12 is contacted with a stopper 14, and thereafter the control and operation are performed under this state. When a power source is turned OFF, and the current Ii becomes zero, a negative voltage E1, which is charged in a capacitor Ca, is applied to the flapper 2. The flapper 2 is displaced in the separating direction from the nozzle 3. The diaphragm 12, which is contacted with the stopper 14 is displaced in the separating direction from the stopper 14. The output voltage can be reduced to zero at the relatively early time from the time when the power source is turned OFF.