To prevent an unassumed external force from being applied to a magnetostriction actuator.

The magnetostriction actuator is provided with a magnetostrictor 11, a coil 12 for applying a magnetic field to thereby displace the magnetostrictor 11, a shaft 13 that is driven in an axial direction by the extraction and contraction of the magnetostrictor 11 corresponding to the application of the magnetic field, and a cover 18 that partially covers an output terminal 13c of the shaft 13. The cover 18 has an opening 18a, and a position A of the opening 18a in an axial direction is set between a position B of a diameter reduction start part 13cs in the axial direction, where the diameter of the shaft 13 starts to be small toward a point 13e and a position C of the point 13e in the axial direction. Thus, when the output terminal 13c of the shaft 13 is brought into close contact with a member to be a diaphragm, an unassumed external force is hardly applied to the shaft because an angle formed between the surface of the member and the axis of the shaft 13 is limited by the cover 18.