A communication system for transmitting and receiving communication data together with signature data attached thereto for verifying the communication data. A transmission-side in-vehicle device of the system generates the signature data for each unit of communication data consisting of M×N (M>=N>=2) pieces of communication data, and repeatedly transmits M pieces of divided signature data in N rounds, attached to M×N corresponding pieces of communication data. A reception-side in-vehicle device of the system reconstitutes the unit of communication data from M×N pieces of received communication data, reconstitutes the signature data from M pieces of received divided signature data, and then verifies the reconstituted unit of communication data with the reconstituted signature data. This can prevent data missing of the signature data due to communication errors to thereby reliably verify the communication data.