Abstract: In a communication system, an in-vehicle relay device relays data between ECUs, which are each connected to a communication line in a vehicle, by communicating with each of the ECUs. Data received by a communication device from a server outside the vehicle is input to an out-of-vehicle relay device. Data to be transmitted by the communication device to the server is output to the communication device by the out-of-vehicle relay device. The out-of-vehicle relay device relays data between the server and the ECUs by passing data to and from the in-vehicle relay device. The out-of-vehicle relay device outputs, to the in-vehicle relay device, data input to the out-of-vehicle relay device, or associated data that is associated with data that was output. The in-vehicle relay device determines whether the relaying performed by the out-of-vehicle relay device is to be suspended, based on associated data output by the out-of-vehicle relay device.