A magnetostrictive actuator has magnetostrictive element having a magnetostrictive, a driving rod moving in a displacement direction of the magnetostrictive element, which is connected with the magnetostrictive element and made of a ferromagnet, a solenoid coil applying magnetic field to the magnetostrictive element, which is arranged around the magnetostrictive element, an enclosure enclosing the magnetostrictive element and the solenoid coil. In the enclosure, a permanent magnet for applying static magnetic bias field to the magnetostrictive element and tubular cases including a ferromagnet are arranged around the enclosed magnetostrictive element, and the driving rod is arranged in the enclosure with a space. Magnetic pull-in force caused between the driving rod and the enclosure can apply prestress to the magnetostrictive element.