In a vehicle network system, a plurality of ECUs are network-connected. The plurality of ECUs include a first ECU that has set therein a secret key from among the secret key and a public key that form a pair and are set on the basis of initialization processing performed when the vehicle network system is created, and a second ECU that has set therein the public key. The second ECU adds, to a transmission signal, an authentication keyword created from the public key and information capable of specifying the second ECU and transmits the transmission signal with the authentication keyword added thereto to the network. The first ECU acquires the authentication keyword and estimates the reliability of the communication signal on the basis of the acquired authentication keyword and the secret key.