A piezoelectric/electrostrictive actuator, including a piezoelectric/electrostrictive element that includes at least one laminate including one piezoelectric/electrostrictive layer and one pair of electrodes disposed respectively on both sides of said piezoelectric/electrostrictive layer and has a moving part corresponding to a portion where said piezoelectric/electrostrictive layer is sandwiched by one pair of said electrodes and a non-moving part corresponding to a portion where said piezoelectric/electrostrictive layer is not sandwiched by one pair of said electrodes, and a moisture proof membrane disposed to cover at least the proximity of the boundary between said moving part and said non-moving part and consisting of silicone-based organic insulation material with storage elastic modulus of 100 kPa or less obtained by dynamic viscoelastic measurement using a parallel cone with diameter of 25 mm under a condition with strain of 0.1%, frequency of 1 Hz, gap of 0.5 mm and temperature of 22° C.