A vehicle network authentication system in which communication data is transmitted and received by use of communication specified based on a communications protocol of a vehicle network, the vehicle network authentication system comprising: a data dividing section that is configured to divide authentication data for authenticating a communication target; a dummy data addition section that is configured to add dummy data for a dummy representation of the divided authentication data, the dummy data being used as data to be transmitted to the vehicle network together with the authentication data; and an authentication section that is configured to verify the divided authentication data and the dummy data by use of a plurality of pieces of verification data for verifying the divided authentication data, wherein the vehicle network is provided with a plurality of vehicle control devices including a first vehicle control device and a second vehicle control device, the first vehicle control device authenticating the second vehicle control device, the data dividing section and the dummy data addition section are provided in the second vehicle control device whereas the authentication section is provided in the first vehicle control device, wherein the authentication section is configured to obtain an authentication rate by at least one of i) dividing a number of pieces of authentication data that have succeeded in authentication by a division number of the authentication data and ii) dividing a data amount of authentication data that have succeeded in authentication by a total data amount of the authentication data, and the authentication section is configured to increase functions capable of being used by the second vehicle control device according to the authentication rate. a data dividing section that is configured to divide authentication data for authenticating a communication target; a dummy data addition section that is configured to add dummy data for a dummy representation of the divided authentication data, the dummy data being used as data to be transmitted to the vehicle network together with the authentication data; and an authentication section that is configured to verify the divided authentication data and the dummy data by use of a plurality of pieces of verification data for verifying the divided authentication data, wherein the vehicle network is provided with a plurality of vehicle control devices including a first vehicle control device and a second vehicle control device, the first vehicle control device authenticating the second vehicle control device, the data dividing section and the dummy data addition section are provided in the second vehicle control device whereas the authentication section is provided in the first vehicle control device, wherein the authentication section is configured to obtain an authentication rate by at least one of i) dividing a number of pieces of authentication data that have succeeded in authentication by a division number of the authentication data and ii) dividing a data amount of authentication data that have succeeded in authentication by a total data amount of the authentication data, and the authentication section is configured to increase functions capable of being used by the second vehicle control device according to the authentication rate.