A communication device includes: a reception antenna; a wireless communication element that synchronizes with a reception signal in an electric wave input from the reception antenna, and receives the reception signal; and a wire communication element connected to the wireless communication element and connected to an external communication device via a communication cable so that the wire communication element transmits a signal to the communication cable. The wire communication element determines whether the wireless communication element synchronizes with the reception signal. When the wire communication element determines that the wireless communication element synchronizes with the reception signal, the wire communication element restricts transmitting the signal to the communication cable.