A communication network system in which a first electronic control unit configured to transmit a message is connected to a second electronic control unit configured to receive the message, the communication network system comprising a count-value notification node configured to periodically transmit a count-value notification message used to notify a count value, which is used to generate and check a message authentication code for the message, to the first electronic control unit and the second electronic control unit, the first electronic control unit and the second electronic control unit being each configured to store its count value and a session key shared by the first electronic control unit and the second electronic control unit, said count-value notification node serving as the first electronic control unit configured to transmit the count-value notification message to the second electronic control unit, wherein the first electronic control unit further comprises an encryption part configured to encrypt a first count value stored in the first electronic control unit using the session key, a MAC generator configured to generate a first message authentication code using the first count value and the session key, and a transmitter configured to periodically transmit the count-value notification message including the encrypted count value and the first message authentication code, and wherein the second electronic control unit further comprises a receiver configured to receive the count-value notification message from the first electronic control unit, a decryption part configured to decrypt, by using the session key, the encrypted count value into a second count value, and a MAC checking part configured to generate a second message authentication code using the second count value and the session key and to thereby check whether the first message authentication code matches the second message authentication code, thus determining: (i) when matched, the second electronic control unit updates its count value stored therein with the second count value corresponding to the first count value, and (ii) when unmatched, the second electronic control unit discards the second count value. an encryption part configured to encrypt a first count value stored in the first electronic control unit using the session key, a MAC generator configured to generate a first message authentication code using the first count value and the session key, and a transmitter configured to periodically transmit the count-value notification message including the encrypted count value and the first message authentication code, and wherein the second electronic control unit further comprises a receiver configured to receive the count-value notification message from the first electronic control unit, a decryption part configured to decrypt, by using the session key, the encrypted count value into a second count value, and a MAC checking part configured to generate a second message authentication code using the second count value and the session key and to thereby check whether the first message authentication code matches the second message authentication code, thus determining: (i) when matched, the second electronic control unit updates its count value stored therein with the second count value corresponding to the first count value, and (ii) when unmatched, the second electronic control unit discards the second count value. (i) when matched, the second electronic control unit updates its count value stored therein with the second count value corresponding to the first count value, and (ii) when unmatched, the second electronic control unit discards the second count value.