An antenna unit equipped with a satellite radio tuner and a tuner-unequipped head unit are connected to each other through a serial bus cable. Equipped in the antenna unit, the satellite radio tuner has a tuner portion, a digital demodulating portion, an interface portion, and a CPU. The interface portion serves as a speech/serial converting portion for converting a digital speech signal into a serial signal. The head unit includes an interface portion for receiving the serial signal sent from the antenna unit through a serial bus cable. The interface portion serves as a serial/speech converting portion for converting the received serial signal into the speech signal.