An active headrest includes a panel inside a seat frame, a panel link supporting the panel and rotating in response to raising of the panel, and a frame which is raised by the panel. A self locking device of the active headrest includes: a guide cylinder in the seat back frame, defining a receiving indentation therein; a return spring in the receiving indentation; and a locking piston in the receiving indentation, a first end of the locking piston being coupled to the return spring, and a second end of the locking piston being exposed to outside the guide cylinder, wherein the locking piston is kept in the guide cylinder by the panel link at the non-operated position of the active headrest, and pressed outside of the guide cylinder by the return spring to restrain rotation of the panel link at the operated position of the active headrest.