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

CONSTITUTION: An actuator 1 mainly comprises an actuator element 20 where magnetic fluid 21 is sealed in elastic film material, a spring 20, wires 26, 27 for coupling them and a slider 6 coupled to the way of the wires 26, 27. Magnetic field is applied through an excitation coil 35 onto the actuator element 20. The actuator element 20 tries to take a spherical shape through surface tension under non-excited state, but elongates in the direction of magnetic field when a magnetic field is applied and stabilized at a position where it is balanced with shrinking force of the spring 17. Consequently, the slider 6 can stop at a proper position corresponding to the strength of exciting current.