An onboard communication system includes a center device that transmits update data as a distribution package by a streaming manner to an electronic control unit installed in a vehicle, a master device that is installed in the vehicle and receives the distribution package and transfer the update data to the electronic control unit, and the electronic control unit that writes the update data transferred from the master device to a storage unit. The center device also transmits a hash value calculated for the update data to the master device. The electronic control unit calculates a hash value for the update data and transmits the hash value to the master device. The master device compares the hash value transmitted from the center device with the hash value transmitted from the electronic control unit to verify integrity of the update data.