A method and apparatus for embedding a watermark in an information signal bit-stream are described. The feature extractor is configured to determine at least one obvious feature in an image frame. The electrode support includes a fixing portion fixed with one end of the belt, and a holder for holding a portion closer to another end of the belt in a movable manner and including a wrapping length adjustment mechanism for adjusting the wrapping length thereof. A non-porous gas supply hose is connected to the porous tube. The two temporary fixing pieces are inserted through an opening in a mirror mounting portion of a vehicle, and the pawls of the temporary fixing pieces are locked to facing edges of the opening to temporarily fix the mirror base to the mirror mounting portion.