An antenna extension device includes a first part disposed on an inner surface of a panel; a second part disposed in space between an opening of a building and the panel, the opening correspondingly accommodating the panel; and a third part disposed on an outer surface of the panel. Two ends of the second part are connected to the first part and the third part respectively to form a U-shape cross-sectional profile, and the third part is connected to a feed point of a sensing device.