A vehicle network system is configured such that a plurality of ECUs that allow internal data to be modified are connected to a network so as to be communicable with one another. The system determines whether the internal data of a first ECU has been modified on the basis of a comparison between combined data and a check value. The combined data is created by collecting and combining pieces of split data, split from original data generated on the basis of the internal data of the first ECU and distributed and held in the plurality of ECUs. The check value is generated on the basis of internal data held in the first ECU at each point in time.