PURPOSE: To avoid drift due to temperature, to reduce the size and to decrease the number of parts by detecting the displacement of a spool where an output member of a piezoelectric actuator having a piezoelectric element and a displacement magnifying device is engaged, and feedbacking the detected displacement to an operation control portion.

CONSTITUTION: A spherical portion of the forward end of an output member 12 provided on a displacement magnifying device 12 of a piezo electric actuator 3 is engaged with a groove of a spool 7 without axial play, and the displacement of the spool 7 is detected by a transformer 24. When the driving voltage of a piezoelectric element 14 is the half of the maximum applied voltage, the spool 7 is to be put in a neutral position, and a signal of the differential transformer 24 is feedbacked to a driving electric circuit of the piezoelectric element 14. Accordingly, the spool 7 is driven directly by the piezoelectric actuator 3, the spool is strong against contaminant and has good responsiveness, and drift due to fluid temperature is avoided to reduce power loss, so that the valve can be reduced in size and the number of parts can be decreased.