An antenna device shared by three different waves has a single two-stage collinear antenna constituted by half-wave dipole antennas which are adapted for a vehicle telephone signal and are stacked one upon the other. The device allows an AM-FM radio receiver to receive AM and FM broadcast signals via a first coaxial cable and a vehicle telephone transceiver to receive a telephone signal via a second coaxial cable. A first impedance converting circuit is implemented as a field effect transistor and connected to the base end of the antenna for matching the antenna and the first coaxial cable in the event of reception of the AM broadcast signal. A second impedance converting circuit matches the antenna and the first coaxial cable at the time of reception of the FM broadcast signal. The impedance converting circuits convert the input/output impedances of the AM and FM signals. The antenna has an antenna rod extending from the base end thereof, a feed tube electrically connected to the antenna rod, and an adjusting feed tube electrically connected to the feed tube and slidably mounted at one end thereof to one the terminating end of the feed tube, and formed with a feed point at the other end. The adjusting feed tube is adjustable in position in its sliding direction to adjust the input/output impedance of the telephone signal as measured at the feed point.