A controlling device of a tunable filter includes a base, a tunable filter on the base along an optical path between first and second optical fibers, a holder rotatably installed on the base to support the tunable filter, a driving unit installed on the base for providing a rotational force, a speed reducing device, and a rotating unit for rotating the holder in accordance with the linear movement of the movable member. The speed reducing device has a worm gear extending from a rotating shaft of the driving unit, and a movable member provided with a threaded hole engaged with the worm gear to allow reciprocal movement along the worm gear according to the rotation direction of the worm gear.