PURPOSE: To realize an actuator having simple structure, by utilizing feature of magnetic fluid to be elongated in the direction of magnetic field to be applied thereon.

CONSTITUTION: An actuator body 1 is constructed with actuator elements 20a, 20b where magnetic fluid 21a, 21b is sealed in an elastic film member with a slider 6 being held through springs 17a, 17b. The actuator elements 20a, 20b are applied with magnetic field through exciting coils 33a, 33b. The actuator elements 20a, 20b tries to take a spherical shape through surface tension under non-excited state, and elongates in the direction of magnetic field when the magnetic field functions and stops at a position where it is balanced with the forces of springs 10a, 17b. Consequently, the position of the slider 6 can be varied corresponding to the level of current fed through respective exciting coils 33a, 33b.