To provide a communication system which can perform a communication surely even if a vehicle is moved when a narrow-band radio communication is performed and to provide an onboard device, a road-side device and a recording medium.

In Step 100, the number of times of retrials of a communication with a first road-side antenna A1 in a communication state at a present is detected. in Step 110, the number of times of retrials of a communication with a second road-side antenna A2 is detected. In Step 120, whether the number of times of the retrials of the communication with the antenna A1 is at a prescribed value (a) or more is decided. In Step 150, the communication state at present is decided to be not worsened, and the communication with the antenna A1 is maintained. In Step 130, the communication state is decided to be worsened, and whether the number of times of the retrials of the communication with the antenna A2 is at a prescribed value (b) or more is decided. In Step 140, since the communication state of the antenna A2 is better than that of the antenna A1, a communication partner is changed over to the antenna A2.