In a fraud-detection method for use in an in-vehicle network system including a plurality of electronic control units (ECUs) that exchange messages on a plurality of buses, a plurality of fraud-detection ECUs each connected to a different one of the buses, and a gateway device, a fraud-detection ECU determines whether a message transmitted on a bus connected to the fraud-detection ECU is malicious by using rule information stored in a memory. The fraud-detection ECU transmits an error message including a message identifier of a message determined to be malicious. The gateway device receives updated rule information transmitted to a first bus among the buses, selects a second bus different from the first bus, and transfers the updated rule information only to the second bus. A fraud-detection ECU connected to the second bus acquires the updated rule information and updates the rule information stored therein by using the updated rule information.