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

CONSTITUTION: An actuator body 1 comprises actuator elements 20a, 20b where magnetic fluid 21a, 21b is sealed in an elastic film member, a wire 28 for coupling the actuator elements 20a, 20b through springs 24a, 24b and a pulley 29 to be rotated through the wire 28. Magnetic field is applied through exciting coils 35a, 35b onto the actuator elements 20a, 20b. The actuator elements 20a, 20b try to take spherical shapes through surface tension under unexcited state, but elongated in the direction of magnetic field when they are subjected to the magnetic field and stop at positions where they are balanced with the springs 24a, 24b. Consequently, rotary position of the pulley 29 can be varied corresponding to the level of current to be fed to the exciting coils 35a, 35b.