A vehicle network system, comprising: a plurality of control units provided on a vehicle and network-connected to each other communicatively, the plurality of control units including: a first control unit that has set therein a secret key from among the secret key and a public key that form a pair and are set by execution of initialization processing for activating the system; and a second control unit that has set therein the public key, wherein the first control unit is configured to transmit a communication signal to the second control unit via the network and also transmit an authentication signal created on the basis of the secret key and original authentication data generated on the basis of the communication signal; and the second control unit is configured to receive the communication signal and the authentication signal and estimate reliability of the communication signal on the basis of comparison of reproduced authentication data generated on the basis of the received communication signal and decoded authentication data decoded on the basis of the authentication signal and the public key, characterized in that the first control unit is configured to add a first identifier to the communication signal, the first control unit is configured to add a second identifier to the authentication signal, the second control unit is configured to further estimate reliability on the basis of the state where the first identifier and the second identifier are the corresponding identifiers, the plurality of control units are configured to communicate with each other by means of a signal constituted by a format of CAN protocol. a plurality of control units provided on a vehicle and network-connected to each other communicatively, the plurality of control units including: a first control unit that has set therein a secret key from among the secret key and a public key that form a pair and are set by execution of initialization processing for activating the system; and a second control unit that has set therein the public key, wherein the first control unit is configured to transmit a communication signal to the second control unit via the network and also transmit an authentication signal created on the basis of the secret key and original authentication data generated on the basis of the communication signal; and the second control unit is configured to receive the communication signal and the authentication signal and estimate reliability of the communication signal on the basis of comparison of reproduced authentication data generated on the basis of the received communication signal and decoded authentication data decoded on the basis of the authentication signal and the public key, characterized in that the first control unit is configured to add a first identifier to the communication signal, the first control unit is configured to add a second identifier to the authentication signal, the second control unit is configured to further estimate reliability on the basis of the state where the first identifier and the second identifier are the corresponding identifiers, the plurality of control units are configured to communicate with each other by means of a signal constituted by a format of CAN protocol. a first control unit that has set therein a secret key from among the secret key and a public key that form a pair and are set by execution of initialization processing for activating the system; and a second control unit that has set therein the public key, wherein the first control unit is configured to transmit a communication signal to the second control unit via the network and also transmit an authentication signal created on the basis of the secret key and original authentication data generated on the basis of the communication signal; and the second control unit is configured to receive the communication signal and the authentication signal and estimate reliability of the communication signal on the basis of comparison of reproduced authentication data generated on the basis of the received communication signal and decoded authentication data decoded on the basis of the authentication signal and the public key, characterized in that the first control unit is configured to add a first identifier to the communication signal, the first control unit is configured to add a second identifier to the authentication signal, the second control unit is configured to further estimate reliability on the basis of the state where the first identifier and the second identifier are the corresponding identifiers, the plurality of control units are configured to communicate with each other by means of a signal constituted by a format of CAN protocol.