To provide a magnetostrictive actuator movable by a small power consumption and to provide an optical switch using the magnetostrictive actuator.

The magnetostrictive actuator includes a magnetostrictive element piece of the magnetostrictive actuator in which a chromium layer for generating a reverse warp deformation to the magnetostrictive material layer is formed between a substrate and the magnetostrictive material layer to thereby cancel the warp deformation of the magnetostrictive element piece generated at a deposition time by a gas phase method. Thereby, in the state that a magnetic field is not applied, the magnetostrictive element piece which does not generate a warpage can be obtained. Moreover, in the magnetostrictive actuator using such a magnetostrictive element piece, since a size (area) of a magnetic field generating part can be designed small by considering only an amount of the maximum deflection of the magnetostrictive element piece by applying the magnetic field, the magnetostrictive actuator having a small power consumption can be obtained.