A disclosed first communication device includes a communication mode detecting part for detecting a predetermined communication mode with respect to a second communication device, a communication mode setting part for setting a communication mode of the first communication device, and a jam signal detecting part for detecting a jam signal from the second communication device. The communication mode setting part is configured to set the communication mode of the first communication device to a full-duplex communication mode when the predetermined communication mode detected by the communication mode detecting part is a half-duplex communication mode, and configured to set the communication mode of the first communication device to the half-duplex communication mode when the jam signal is detected by the jam signal detecting part.