An anti-fraud method for use in an in-vehicle network system including a plurality of electronic control units that exchange data frames, each having added thereto a message authentication code (MAC), via at least one bus includes: receiving a data frame transmitted on the bus; generating a first MAC by using a MAC key and a value of a counter that counts the number of times a data frame having added thereto a MAC is transmitted; in a case where the verification has failed, (i) generating as second MAC by using an old MAC key; (ii) re-verifying that the received data frame has added thereto the generated second MAC; transmitting, in a case where the re-verification has succeeded, via the bus a key-update frame indicating a request for updating the MAC key; and updating the MAC key in response to the transmission of the key-update frame.